Release the merge request diff replication

What does this MR do?

Remove the feature flag geo_merge_request_diff_replication and release the geo replication for merge request diffs.

Fixes #247100 (closed)

Screenshots

Files on the Primary:

root@alexives-primary-geo:/var/opt/gitlab/gitlab-rails/shared/external-diffs# tree .
.
├── merge_request_diffs
│   └── mr-1
│       └── diff-1
└── tmp
    ├── cache
    └── work

5 directories, 1 file

Files on the Secondary:

root@alexives-secondary-geo:/var/opt/gitlab/gitlab-rails/shared/external-diffs# tree .
.
└── merge_request_diffs
    └── mr-1
        └── diff-1

2 directories, 1 file

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