Release v15.8.0 and Upgrade to GitLab 15.8

This change set prepares the %15.8 provider release.

Todo:

• Verify everything that changelog is complete
  • Check status of !1425 (merged) (should go into %15.8 )
  • Check status of !1427 (merged) (should go into %15.8 )
  • Check status of !1428 (merged) (should go into %15.8 )
  • [ ] Check status of !1419 (merged) (may go in if by "coincident" it's ready)
• Upgrade test matrix with latest releases from %15.8 (including bug fix releases of %15.6 and %15.7)
• Verify everything is still working with the new releases