Re-enable downgrade tests after dependency updates

[ChrisRackauckas-Claude]

Summary

The downgrade CI tests have been temporarily disabled while waiting for dependency updates to stabilize.

Action Items

1. Wait for necessary dependency updates to be released
2. Remove the if: false conditional from .github/workflows/Downgrade.yml
3. Verify that the downgrade tests pass

Related

This follows the same pattern as SciML/DelayDiffEq.jl#356.