Enable caching for milestone issue counters by default

What does this MR do?

We want to expose milestone issue counts to the API for #31289 (closed).

The current milestones page is also very slow - it has N+1 for the same counters.

We want to cache these counters, but they depend on a user, which makes cache useless.

1. !24284 (merged) introduced caching hidden behind a feature flag
2. This MR enables this caching by default and also removes user variable from all related calculations
3. !19937 (merged) will expose those counters in the API

This is still WIP until I enable a feature flag in production, see performance improvement and wait for some time to ensure we didn't introduce any problems

Related discussion: #31289 (comment 258765932)

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

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