Stop building SLES 15.2 packages

SLES 15.2 is EOL since December 2024 and we already support SLES 15.6. We can deprecate SLES 15.2 builds.

Acceptance Criteria

in %17.9

• announce the deprecation in gitlab-org/gitlab gitlab!178842 (merged)
• add a deprecating warning to the package !8136 (merged)

in %18.0

• SLES 15.2 jobs removed !8524 (merged)
• SLES 15.2 moved to deprecated OSes gitlab!196264 (merged)
• SLES 15.2 builder image removed and docs updated. gitlab!196264 (merged)