Stop building openSUSE Leap 15.5 packages

openSUSE Leap 15.5 reaches EOL December 2024 as per https://en.opensuse.org/Lifetime#openSUSE_Leap.

Tasks:

1. Add deprecation warning
2. Announce deprecation in next release post
3. Remove Leap 15.5 jobs from pipeline !8116 (merged)
4. Remove from supported operating systems page / Add to no longer supported list gitlab!177873 (merged)
5. Remove from install page gitlab-com/marketing/digital-experience/buyer-experience#4346 (closed)
6. Drop builder image gitlab-omnibus-builder!370 (merged)