Code suggestions - Keep opt-in, do not enable by default, expand to premium.

In this morning's AI Standup, Sid and e-group decided to not move forward with the migration to enable by default which was requested in the standup yesterday. #407766 (closed)

• We are leaving Code Suggestions, off-by default. (this is how it works prior to yesterday's changes)
• We have closed the settings changes MR !118057 (closed)
• We are reverting this (!117895 (merged)) back to opt-in by default. REVERT MR !118073 (merged)
  • Note after this revert here's how this will look:
    • Existing customers - off by default
    • newly signed up customers since yesterday - on by default (due to !117895 (merged))
    • new customers moving forward - off by default
• We will not run the migration for enabling this for all ultimate namespaces introduced with !117899 (merged)
• We declared an incident to prevent the above migration from deploying and being run https://gitlab.com/gitlab-com/gl-infra/production/-/issues/8752
• We will revert the migration change from yesterday - REVERT MR !118070 (merged)
• We will expand Code suggestions to premium namespaces. !118130 (merged)
• Annouce in release post in %15.11 gitlab-com/www-gitlab-com!122369 (574f7c0a)

See https://gitlab.slack.com/archives/C048Z2DHWGP/p1681920007577779