Hide instance-level integrations on GitLab.com

What does this MR do?

Issue #238570 (closed)

Instance-level integrations should not be used on GitLab.com (we have group-level integrations for similar functionality) so we should hide it to prevent errors or abuse.

• The Admin Area > Settings > Integrations menu item is removed on GitLab.com.
• The instance-level integration listing and edit pages return 404 Not Found when accessed on GitLab.com

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