Re-enable downgrade tests

[ChrisRackauckas-Claude]

Summary

The downgrade tests have been disabled in the CI workflow pending dependency updates.

Background

The downgrade CI tests have been consistently failing due to compatibility issues with lower bounds of dependencies. This is similar to what was done in other SciML packages (e.g., DelayDiffEq.jl #356).

Action Items

• Investigate which dependency lower bounds need updating
• Update the compat entries in Project.toml
• Re-enable the downgrade tests by removing if: false from .github/workflows/Downgrade.yml

Related

• PR Fix CI by removing unused isinplace import #57 disabled the downgrade tests