Clear hard failure when updating mirror via UI

What does this MR do?

There was a regression wherein updating a mirror that hard failed wasn't updating even when the update is triggered via web UI.

It was because the StartPullMirroringService was reseting the retry count before calling ProjectImportState#force_import_job!. As a result, the method will return and do nothing because it detects that the mirror is due to be updated and it's no longer considered as a hard failure.

To fix it, we remove the need to reset the retry count before #force_import_job! is called. The retry count will be reset in #force_import_job! once it deemed that the mirror can be force updated.

Now, we only reset the retry count when updating the next execution timestamp in the said service.

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

Closes #221321 (closed)