Remove non ldap user after the lock to ldap is set for groups

What does this MR do?

This MR adds a service that will response to the change of the boolean flag unlock_membership_to_ldap of a group, the service will enqueue a job to check for non ldap users and if needed it will remove them from the given group and its descendants (subgroups and projects).

This is related to: #1793

This is behind :ldap_settings_unlock_groups_by_owners feature flag

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