[Feature flag] Enable Geo Merge Request Diff replication

<!-- Title suggestion: [Feature flag] Enable description of feature --> ## What Remove the `:geo_merge_request_diff_replication` feature flag ... ## Owners - Team: ~"group::geo" - Most appropriate slack channel to reach out to: `#g_geo` - Best individual to reach out to: `@toon` ## Expectations ### What are we expecting to happen? ### What might happen if this goes wrong? ### What can we monitor to detect problems with this? <!-- Which dashboards from https://dashboards.gitlab.net are most relevant? --> ## Beta groups/projects If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example. - `gitlab-org/gitlab` project - `gitlab-org`/`gitlab-com` groups - ... ## Roll Out Steps - [ ] Enable on staging - [ ] Test on staging - [x] Test in GCP (e.g. with instances spun up with ansible-gitlab-geo) - [x] Ensure that documentation has been updated - [ ] Enable on GitLab.com for individual groups/projects listed above and verify behaviour - [ ] Coordinate a time to enable the flag with `#production` and `#g_delivery` on slack. - [ ] Announce on the issue an estimated time this will be enabled on GitLab.com - [ ] Enable on GitLab.com by running chatops command in `#production` - [ ] Cross post chatops slack command to `#support_gitlab-com` ([more guidance when this is necessary in the dev docs](https://docs.gitlab.com/ee/development/feature_flags/controls.html#where-to-run-commands)) and in your team channel - [ ] Announce on the issue that the flag has been enabled - [x] Remove feature flag and add changelog entry - [ ] After the flag removal is deployed, [clean up the feature flag](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up) by running chatops command in `#production` channel

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