Feature flag for variables with potentially long values

What does this MR do?

This MR places a feature flag in front of two CI variables that are not yet functional but have potential performance issues once they are. There's a thorough discussion of the performance and abuse concerns here !31038 (comment 339351278) but for the time being this should be implemented to be safe.

The variables were originally introduced in 12.9, but there's an outstanding fix required to use them #212243 (closed) so putting in the feature flag shouldn't break any workflows.

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