Re-enable downgrade CI tests

[ChrisRackauckas-Claude]

Summary

The downgrade CI tests have been disabled in PR #521 because the JET test dependency is incompatible with the downgraded environment.

Details

JET is listed as a test dependency in Project.toml:

[targets]
test = ["Aqua", "FastBroadcast", "ForwardDiff", "JET", ...]

When running with downgraded dependencies, Pkg cannot resolve JET because it has strict Julia version requirements that conflict with the downgrade environment.

Action Items

• Monitor JET releases for improved compatibility
• Consider moving JET to a separate test environment to avoid resolution conflicts
• Re-enable downgrade tests once the dependency conflict is resolved

Related

• PR Move JET tests to nopre group #521 disabled the downgrade tests