Fix annotations actions and feature flag

What does this MR do?

The feature flag name was incorrectly used in the frontend metrics_dashboard_annotations -> metricsDashboardAnnotations. This MR fixes it.

Also, this MR removes REQUEST_ANNOTATION mutation type as it does not update the state. Such actions throw the following console error

Note: All of this is still behind metrics_dashboard_annotations feature flag.

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

Mentions: #211418 (closed)