feat: remove experimentals for modular models/schema 1.2

[ewanharris]

Description

Removes the experimental flag that was guarding the ability to write modular models/schema 1.2.

I've left in some of the tests just to exercise that the ability to write 1.2 is there but happy to remove if that's perferred.

References

Original PR #1443

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.08%. Comparing base (ab0d835) to head (9172176).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1520      +/-   ##
==========================================
+ Coverage   86.07%   86.08%   +0.02%     
==========================================
  Files          85       85              
  Lines        8046     8038       -8     
==========================================
- Hits         6925     6919       -6     
+ Misses        790      789       -1     
+ Partials      331      330       -1     

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[jon-whit]

Merging despite the govulncheck report. Will follow up with that in a separate PR as it is unrelated.