[Feature flag] Roll out :paginated_notes feature

What

Remove the :paginated_notes feature flag ...

This one is an interesting beast.

We want to remove the discussions.json endpoint in favour of the notes.json endpoint, since the former is slow and hard to paginate, while the latter is fast(er) and easy to paginate: #209784 (closed). So we're introducing pagination of the latter.

Technically we can introduce the pagination separately to this effort. However, we've come across a few issues while trying to get there, so we can't remove this feature flag yet. Here's some dependent issues:

• #250573
• #254627 (closed)

and dependent MRs:

• !34628 (merged)
• !42226 (closed)
• !42525 (closed)
• !43015 (merged)

Owners

• Team: groupsource code
• Most appropriate slack channel to reach out to: #g_create-source_code_be
• Best individual to reach out to: @nick.thomas

Expectations

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

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 (/chatops run feature set feature_name true --staging)
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab feature_name true)
• 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 (/chatops run feature set feature_name true)
• Cross post chatops Slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel