Update merge options for auto merge strategies

What does this MR do?

After Auto Merge is enabled on a merge request, users can update merge params on the merge, for instance, they can update a source branch deletion option from false to true.

However, the update request goes through #execute method, users cannot update merge params if the merge request doesn't fullfill #available_for criteria.

We should let users update merge params whenever Auto Merge is enabled on a merge request.

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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