v15.5.0

## Description

### Automatic Updates

15.5 introduces a new automatic update mechanism for system administrators to control which Teleport version their
agents are running. You can now configure the agent update schedule and desired agent version via the `autoupdate_config`
and `autoupdate_version` resources.

Updates are performed by the new `teleport-update` binary.
This new system is package manager-agnostic and opt-in. Existing agents won't be automatically enrolled, you can enroll
existing 15.5+ agents by running `teleport-update enable`.

`teleport-update` will become the new standard way of installing Teleport as it always picks the appropriate Teleport
edition (Community vs Enterprise), the cluster's desired version, and the correct Teleport variant (e.g. FIPS-compliant
cryptography).

You can find more information about the feature in [our documentation]().

### Package layout changes

Starting with 15.5.0, the Teleport DEB and RPM packages, notably used by the `apt`, `yum`, `dnf` and `zypper` package
managers, will place the Teleport binaries in `/opt/teleport` instead of `/usr/local/bin`.

The binaries will be symlinked to their previous location, no change should be required in your scripts or systemd units.

This change allows us to do automatic updates without conflicting with the package manager.

### Readiness endpoint changes

The Auth Service readiness now reflects the connectivity from the instance to
the backend storage, and the Proxy Service readiness reflects the connectivity
to the Auth Service API. In case of Auth or backend storage failure, the
instances will now turn unready. This change ensures that control plane
components can be excluded from their relevant load-balancing pools. If you want
to preserve the old behaviour (the Auth Service or Proxy Service instance stays
ready and runs in degraded mode) in the `teleport-cluster` Helm chart, you can
now tune the readiness setting to have the pods become unready after a high
number of failed probes.

### Other fixes and improvements

* Increased the email access plugin timeout for sending e-mails from 5 to 15 seconds. [#54379](https://github.com/gravitational/teleport/pull/54379)
* The `teleport-cluster` Helm chart now supports tuning the pod readiness. [#54351](https://github.com/gravitational/teleport/pull/54351)
* Fixed a potential panic during Auth Server startup when the backend returns an error. [#54325](https://github.com/gravitational/teleport/pull/54325)
* Fix issue that prevent Kubernetes agents from connecting to GKE control plane using the new DNS-based access mechanism. [#54218](https://github.com/gravitational/teleport/pull/54218)
* Teleport-update: stabilize binary paths in generated tbot config. [#54193](https://github.com/gravitational/teleport/pull/54193)
* Reduce log spam in discovery service error messaging. [#54151](https://github.com/gravitational/teleport/pull/54151)
* The web UI now shows role descriptions in the roles table. [#54135](https://github.com/gravitational/teleport/pull/54135)
* Leaf cluster joining attempts that conflict with an existing cluster registered with the root now generate an error instead of failing silently. [#54132](https://github.com/gravitational/teleport/pull/54132)

## Download

Download the current and previous releases of Teleport at https://goteleport.com/download.

## Plugins

Download the current release of Teleport plugins from the links below.
* Slack [Linux amd64](https://cdn.teleport.dev/teleport-access-slack-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-slack-v15.5.0-linux-arm64-bin.tar.gz) 
* Mattermost [Linux amd64](https://cdn.teleport.dev/teleport-access-mattermost-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-mattermost-v15.5.0-linux-arm64-bin.tar.gz)
* Discord [Linux amd64](https://cdn.teleport.dev/teleport-access-discord-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-discord-v15.5.0-linux-arm64-bin.tar.gz)
* Terraform Provider [Linux amd64](https://cdn.teleport.dev/terraform-provider-teleport-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/terraform-provider-teleport-v15.5.0-linux-arm64-bin.tar.gz) | [macOS amd64](https://cdn.teleport.dev/terraform-provider-teleport-v15.5.0-darwin-amd64-bin.tar.gz) | [macOS arm64](https://cdn.teleport.dev/terraform-provider-teleport-v15.5.0-darwin-arm64-bin.tar.gz) | [macOS universal](https://cdn.teleport.dev/terraform-provider-teleport-v15.5.0-darwin-universal-bin.tar.gz)
* Event Handler [Linux amd64](https://cdn.teleport.dev/teleport-event-handler-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-event-handler-v15.5.0-linux-arm64-bin.tar.gz) | [macOS amd64](https://cdn.teleport.dev/teleport-event-handler-v15.5.0-darwin-amd64-bin.tar.gz)
* PagerDuty [Linux amd64](https://cdn.teleport.dev/teleport-access-pagerduty-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-pagerduty-v15.5.0-linux-arm64-bin.tar.gz)
* Jira [Linux amd64](https://cdn.teleport.dev/teleport-access-jira-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-jira-v15.5.0-linux-arm64-bin.tar.gz)
* Email [Linux amd64](https://cdn.teleport.dev/teleport-access-email-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-email-v15.5.0-linux-arm64-bin.tar.gz)
* Microsoft Teams [Linux amd64](https://cdn.teleport.dev/teleport-access-msteams-v15.5.0-linux-amd64-bin.tar.gz) | [Linux arm64](https://cdn.teleport.dev/teleport-access-msteams-v15.5.0-linux-arm64-bin.tar.gz)