Fix params not filtered on project approval API

What does this MR do?

When permissions to modify MR merge approval settings are denied (i.e. project is regulated under compliance label), API request to update these values are ignored. The current implementation has a bug where the global params are mutated but then another instance of params is returned, which results in no-op filtering effect.

This MR is to ensure the method just use the params in the method argument and avoid undesirable mutation of global params. The tests are also refactored to improve coverage.

Screenshots

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