ci: Move two variables from project config to .gitlab-ci.yml

What does this MR do and why?

Post-merge todos:

• Backport this change to the latest 3 stable branches
  • 15.11: !119390 (merged)
  • 15.10: !119393 (merged)
  • 15.9: !119395 (merged)
• Delete the CACHE_ASSETS_AS_PACKAGE and BUNDLER_CHECKSUM_VERIFICATION_OPT_IN variables

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.