Fix: [Actions] Remove Ubuntu:groovy from release pipeline (OpenTTD#9846)

It no longer exists upstream. Let's focus on LTS only for now.

Doc: Prepare for 12.1 release

Change: Title game for 12.0 release

Doc: Prepare for 12.0-RC1 release (OpenTTD#9574)

Doc: Prepare for 12.0-beta2 release (OpenTTD#9489)

Fix: [Actions] Ubuntu 18.04 by default has a compiler too old for Ope…

…nTTD (OpenTTD#9481)

Update: Changelog for 1.11.2 release

Add: Changelog entries for 1.11.1 release

Fix: [CMake] if the regex for STABLETAG is empty, it means it is stable

This feels a bit inside out, but it makes sense: if there are no
"beta1" or "RC1" mentions, it means it is a stable release.

Doc: Prepare for 1.11.0-RC1 release (OpenTTD#8859)