Auto approve users when User Cap setting is increased or set to nil

What does this MR do?

Adds AutoApproveBlockedUsersWorker which will be performed async if the the User Cap setting is increased or set to nil. This worker will call the Users::ApproveService for all the users in blocked_pending_approval state.

If the the User Cap is decreased or was set to nil previously, no users should be auto approved.

🚩 This feature is under the admin_new_user_signups_cap feature flag.

First part of #273258 (closed)

Screenshots (strongly suggested)

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