Improvement to merged_branch_names cache

What does this MR do?

Second iteration for #30536 (closed), following on from !23458 (merged).

This rewrites the cache work to use a Redis hash store. This should result in less work happening in Ruby, but maintains very few cache calls by using pipelining. It also increases the cache time for these values to 2 weeks instead of 10 minutes from the first iteration.

The feature flag is still in place for this, and will be removed in a later MR.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team