Deprecate OpenSUSE 15.4

openSUSE Leap 15.4 will have its EOL on at end of November 2023. In GitLab 16.4 we introduced supported OpenSUSE 15.5 packages.

We should:

• Add deprecation note for the OpenSUSE 15.4 package gitlab!132050 (merged)
• Add deprecation to the omnibus package !7154 (merged)