v4.0.21

## What's Changed
* refactor: update "Included Monitors" label to include count in variou… by @rajnandan1 in https://github.com/rajnandan1/kener/pull/670
* Fixcustom js css guide link by @rajnandan1 in https://github.com/rajnandan1/kener/pull/671
* Update Czech and Slovak translation by @danynocz in https://github.com/rajnandan1/kener/pull/673
* refactor: implement upcoming maintenances feature in dashboard and mo… by @rajnandan1 in https://github.com/rajnandan1/kener/pull/677
* security: fix critical and high severity vulnerabilities by @LMtx in https://github.com/rajnandan1/kener/pull/678
* refactor: support multi-monitor alerts by updating alert configuratio… by @rajnandan1 in https://github.com/rajnandan1/kener/pull/679
* refactor: implement global maintenance notification settings and upda… by @rajnandan1 in https://github.com/rajnandan1/kener/pull/688

## New Contributors
* @LMtx made their first contribution in https://github.com/rajnandan1/kener/pull/678

**Full Changelog**: https://github.com/rajnandan1/kener/compare/v4.0.20...v4.0.21