Cache merge requests count in group sidebar [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Related to #299542 (closed)

Similarly to the work done for issues, and for performance reasons, in this MR we implement caching the merge request count in the sidebar at a group level. This value will be cached with the following conditions:

• The total count will be cached only if it's over the threshold value (currently 1000).
• The cache will expire after 24 hours.
• To reflect that any count over the threshold value is "imprecise" they will be truncated by thousands. For example, 2350 -> 2.3k, 14300 -> 14.3k, 112560 -> 112.6k.

This change is behind the FF cached_sidebar_merge_requests_count, enabled by default.

Screenshots (strongly suggested)

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

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