Remove feature flag `status_page_attachments`

What does this MR do?

This feature flag has been enabled by default since %13.1 and no issues have been found so it's safe to assume it's not broken.

Note that this feature flag was enabled by default and !31269 (merged) already contained the changelog entry. It was also not mentioned in the documentation.

Closes #219286 (closed).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - was enabled by default and !31269 (merged) already contained the changelog entry
• [-] Documentation (if required) - this feature flag was not mentioned in the docs
• 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