Re-enable downgrade tests after OrdinaryDiffEq updates

[ChrisRackauckas-Claude]

Summary

The downgrade tests have been temporarily disabled pending updates to OrdinaryDiffEq. This is consistent with the approach taken in other SciML packages (e.g., DelayDiffEq.jl).

Action Required

Once OrdinaryDiffEq updates are complete:

1. Remove the if: false condition from .github/workflows/Downgrade.yml
2. Verify that downgrade tests pass successfully

Related

• DelayDiffEq.jl PR #357 - similar change