Cleans up multiple_cache_per_job FF and associated code [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

• Removes the multiple_cache_per_job feature flag
• Splits up the cache entries: Entry::Cache and Entry::Caches
• Updates Entry::Default, Entry::Root and Entry::Job to use Entry::Caches instead of singular
• Cleans up specs

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

#321877 (closed)