Rollout plan for critical projects 2FA requirement

[di]

Breaking this out from #11625. The following steps should be followed to roll out the critical projects 2FA requirement:

• Resolve Mandatory 2FA with existing users without 2FA vulnerable to account takeover #11850
• Evaluate how many critical project maintainers will be affected by enabling the mandate.
  • Ideally this is non-zero, but likely that some projects/maintainers may be unresponsive
  • Depending on the size, potentially do some manual comms to affected maintainers
• Set TWOFACTORMANDATE_ENABLED to True
  • This makes 2FA required for critical projects and completes the rollout
• Tweets/announcements about this milestone