[Feature flag] Rollout of `personal_snippet_reference_filters`
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## 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: \
{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: \
{width=440 height=121}
* Example of it working (shown on staging): \
{width=672 height=345} \
{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?
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## 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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