Debounce pull mirror invocation

What does this MR do?

For the system to handle the load of pull mirrors further, we need to limit the number of pull mirror invocations per project.

It's documented that within the past 5 minutes, user can only manually trigger a pull mirror update once.

Before this change, we are letting the user update a pull mirror immediately. Now, we are checking the last_update_at of the ImportState and see if the mirror was updated in past 5 minutes. If so, schedule the next execution 5 minutes since last update.

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

#118753 (closed)