Only assert backwards-compat experiments

What does this MR do?

This MR fixes a small issue with the original implementation of the spec from !45733 (merged). That spec would require us to either remove it – assuming that it is no longer relevant at all – or to continually update it as we add new experiments, even though new experiments won’t be marked as being backwards compatible.

Also, this way it provides a nice to-do list of experiments that need to be finished & cleaned up. Once the list in the spec is emptied, we can remove the backwards compatible code entirely.

No changelog entry or documentation is needed as this is a purely developer-facing change.

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

Related to #270858 (closed)