Remove feature flag for "Create issue" from alert detail page

What does this MR do?

This removes the feature flag that was hiding alert issue creation functionality on the backend.

This also adds the related documentation.

Related to: #218045 (closed)

Related Todos:

• Ensure that documentation has been updated
• Cross post chatops slack command to #support_gitlab-com ([more guidance when this is necessary in the dev docs]
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

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