Convert auto_disabling_web_hooks ops flag to application setting

Why are we doing this work

The auto_disabling_web_hooks ops feature flag has been in use for an extended period and should be converted to an application setting. Feature flags are meant to be short-lived, and this long-lived ops flag should be replaced with a proper application setting to provide a more stable and maintainable configuration option.

This work addresses the comment in !218726 (merged) where it was identified that ops flags should have a clear path to removal, and converting to an application setting is the appropriate next step.

Relevant links

• Original discussion: !218726 (comment 3076579396)
• Feature flag definition: config/feature_flags/ops/auto_disabling_web_hooks.yml

Non-functional requirements

• Documentation: Update documentation to reflect the new application setting and deprecation of the feature flag
• Feature flag: n/a
• Performance: n/a
• Testing: Add tests for the new application setting and verify backward compatibility during transition

Implementation plan

1. Create a new application setting for auto-disabling webhooks
2. Update the code to check the application setting
   1. the default for gitlab.com is enabled; for everything else it's disabled
3. Plan migration for ops flag into application setting.
   1. Suggestion: if the setting doesn't exist, set it to the value of the feature flag (i.e. either the existing configuration or the default for the deployment type)
4. Update documentation to reflect the new setting
5. Remove the feature flag in a future release

Verification steps

1. Verify that the application setting can be configured via the admin UI
2. Verify that the application setting works as expected when enabled/disabled
3. Verify backward compatibility with the existing feature flag during the transition period
4. Verify that documentation accurately reflects the new configuration method