[Feature flag] Enable event tracking feature flags

What

Remove the :track_resource_milestone_change_events, :track_issue_weight_change_events, :track_resource_state_change_events feature flags

Related to #38096 (closed)

Owners

• Team: groupproject management
• Most appropriate slack channel to reach out to: #s_plan
• Best individual to reach out to: @engwan

Expectations

### What are we expecting to happen?

When milestones, weight, or state (opened / closed / merged) are changed in an issue / MR, we no longer insert to the notes table but insert into the respective event tables.

There should be no visible change to the user since we generate synthetic system notes for these events.

What might happen if this goes wrong?

If something goes wrong, we may see missing system notes in issues / MRs

Roll Out Steps

• Enable on staging
• Test on staging
• 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) 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