Read Only Legacy Feature Flags

What does this MR do?

Adds the code for making legacy feature flags Read-Only.

This flag will not be enabled until %13.4.

Issue: #220228 (closed)

See also: #220227 (closed)

Screenshots

Index page:

• The toggles for legacy flags are disabled.

Tooltip on the index page:

Edit page:

• The message at the top of the screen says the flag is read-only.
• The Save button is disabled.

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