[Feature flag] Rollout of `personal_snippet_reference_filters`
<!-- Title suggestion: [Feature flag] Enable description of feature --> ## What Remove the `:personal_snippet_reference_filters` feature flag ... ## Owners Current: - Team: ~"group::knowledge" - Most appropriate Slack channel to reach out to: `#g_knowledge` - Best individual to reach out to: @ikvikakk Original: - Team: Create::Editor - Most appropriate slack channel to reach out to: `#g_create_editor` - Best individual to reach out to: `@vij` or `@fran` ## Expectations ### What are we expecting to happen? * References to resources added to `PersonalSnippet` comments and descriptions should be converted via the GFM/AbstractReferenceFilter to GitLab references ([docs](https://docs.gitlab.com/user/markdown/#gitlab-specific-references)) on create/update and preview of changes. * Example of such a reference not converted (user FF not enabled) and converted (user FF enabled) in the comments of a personal snippet: \ ![image](/uploads/fd49f977c73542722a0a62c4539f75da/image.png){width=600 height=600} * Links to personal snippets themselves are resolved into `$123` syntax, and the same syntax works to specify them in the first place. * N.B. This requires the FF be enabled site-wide; it doesn't work on a per-actor basis, as reference resolution happens _without respect_ to the viewing user. * Example of it not working: \ ![image](/uploads/edcc24e2e3d283367c20c122632ca43b/image.png){width=440 height=121} * Example of it working (shown on staging): \ ![image](/uploads/bee04053cae99e0a2d6e24b89143add8/image.png){width=672 height=345} \ ![image](/uploads/0d4ec06fd29c22fbdfd44e231bd99b29/image.png){width=403 height=119} ### What might happen if this goes wrong? References may not be generated correctly. ### What can we monitor to detect problems with this? <!-- Which dashboards from https://dashboards.gitlab.net are most relevant? --> ## Beta groups/projects/users ## Roll Out Steps - [x] Enable on staging - [x] Test on staging - [x] Ensure that documentation has been updated - [x] Enable on GitLab.com by running chatops command in `#production` - [x] Announce on the issue that the flag has been enabled - [x] Remove feature flag and add changelog entry - [x] 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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