Update What's new in 6.6 (#21745)

* APT and YUM repositories

* Add julie's fixes

* Fix links

* Docs: Adds backendSrv notice (#21752)

* Docs: Adds backendSrv notice

* Update docs/sources/guides/whats-new-in-v6-6.md

* Add links to installation notes

Co-authored-by: Hugo Häggmark <hugo.haggmark@gmail.com>
Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>