[Feature flag] Rollout of `sse_erb_support`

What

Remove the :sse_erb_support feature flag.

Add feature flag to control ERB syntax support in the Static Site Editor.

Issue: #235176 (closed)

Owners

• Team: Static Site Editor
• Most appropriate slack channel to reach out to: #g_create_static_site_editor
• Best individual to reach out to: @vyaklushin

Expectations

What are we expecting to happen?

We want to enable/disable Static Site Editor support for .md.erb files. From users' perspective they will see an error when opening .md.erb file in Static Site Editor if feature is disabled.

What might happen if this goes wrong?

Worst case scenario - there is a problem with ERB file support in Static Site Editor, and the feature flag does not work. Users can access .md.erb when the feature flag has a false state. It might lead to invalid content in .md.erb file after editing it in Static Site Editor. But users can revert these changes if it happens.

What can we monitor to detect problems with this?

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