v3.21.0

## Fixes

- PrivateVPN: replace special last accented a character with `a` for `Bogota`
- Do not write out servers data and updated timestamp if no change was detected
- (@TJJP) Fix: Windscribe Openvpn config (#528)
  - See [blog.windscribe.com/openvpn-security-improvements-and-changes-7b04](https://blog.windscribe.com/openvpn-security-improvements-and-changes-7b04)
- Server data model version diff:
  - comparison fixed
  - missing "behind" suffix to log line
- Openvpn loop: possible deadlock: unlock read mutex for `GetSettingsAndServers`
- Alpine vulnerability fixed with apk-tools upgraded (except for `ppc64le` architecture)

## Features

- Update all servers for all providers
- Specify Openvpn flags with `OPENVPN_FLAGS`
- Set health timeouts with `HEALTH_OPENVPN_DURATION_INITIAL` and `HEALTH_OPENVPN_DURATION_ADDITION`
- Health server listening address configurable with `HEALTH_SERVER_ADDRESS`
- Updater CLI: add `-all` flag to update all VPN servers

## Breaking changes

- Updater CLI:
  - `-enduser` instead of `-file`
  - `-maintainer` instead of `-stdout`

## Documentation

## Maintenance

### Dependencies

- Upgrade `inet.af/netaddr` to `2021-07-18`
- Upgrade `qdm12/dns` to `v1.11.0`

### CI

- Remove microbadger hook from CI
- Rename `.github/workflows/build.yml` to `.github/workflows/ci.yml` for linting
- Bump `docker/build-push-action` from `2.4.0` to `2.6.1`
- Use curly braces around `BUILDPLATFORM`
- Rename `BUILD_DATE` to `CREATED`
- Build all images fully in parallel (thanks to JSON server information using less memory at compile time)

### Code

- Hardcoded servers data as a JSON embedded file in `internal/constants/servers.json`
  - use `embed.FS` to have immutable data
  - use `sync.Once` to parse JSON data only once without data races
- Remove debug line in health server
- Improve health code
- Use `qdm12/gosplash` for initial logs
- Upgrade `qdm12/golibs` and rework env error wrapping
- Do not mock os functions
  - Use filepaths with /tmp for tests instead
  - Only mock functions where filepath can't be specified such as user.Lookup
- Pass only single strings to logger methods
  - Do not assume formatting from logger's interface
  - Allow to change golibs in the future to accept only strings for logger methods
- Prefer empty string comparisons than `len(s) == 0`
- Firewall and routing use `logger.Debug` instead of `fmt.Println`
- Remove `SetVerbose` and `SetDebug` methods from firewall and routing
- Log routing teardown
- Default logging level set to info
- Inject `command.Cmder` to openvpn and firewall
- Pass network values to firewall constructor
- Move duration formatting to `qdm12/golibs/format`
- `internal/loopstate` package to manage the state of run loops
- Remove routing's `Configurator` from firewall's `Configurator`
- Remove routing's `Configurator` from openvpn's `Configurator`
- Common no custom port forwarding obtention implementation for all providers but PIA

### Packages rework:

- Constructors return concrete structs
- Constructors and struct fields accept narrow interfaces
- Split interfaces in sub-interfaces
- Split Go files in more smaller Go files
- Add interface compilation checks
- Use `internal/loopstate` to manage the state of their Run loop
- Add subpackage `state` to manage the state of the loop thread safely
- Packages reworked:
  - `internal/alpine`
  - `internal/cli`
  - `internal/dns`
  - `internal/firewall`
  - `internal/healthcheck`
  - `internal/httpproxy`
  - `internal/openvpn` (both loop and configurator)
  - `internal/publicip`
  - `internal/routing`