Disable free CI/CD for GitHub for private projects without a paid subscription on March 22

What does this MR do?

#10361 (closed)

Adds a new feature flag, :free_period_for_pull_mirroring, default_enabled: true, that will enable us to disable the CI/CD feature exactly on the 22nd.

To disable the feature, run the following command in the production Slack channel:

/chatops run feature set free_period_for_pull_mirroring false

Follow up issue for clean up: #209787 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team