[Feature flag] Enable design activity events

What

Remove the :design_activity_events feature flag ...

Owners

• Team: ~"group::knowledge"
• Most appropriate slack channel to reach out to: #g_create_knowledge
• Best individual to reach out to: @alexkalderimis

Expectations

### What are we expecting to happen?

Nothing - performance may improve due to better SQL queries without the flag

What might happen if this goes wrong?

Performance may be degraded, we might see a rise in 500 errors if DB records cannot be rendered.

What can we monitor to detect problems with this?

• https://dashboards.gitlab.net/d/rPsQXrImk/rails-controller?orgId=1&refresh=1m
• https://dashboards.gitlab.net/d/rPsQXrImk/rails-controller?orgId=1&refresh=1m&var-env=gprd&var-type=web&var-stage=main&var-controller=DashboardController&var-action=All

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