Re-enable downgrade tests after upstream compat fixes

The downgrade tests (both main package and sublibraries) have been disabled because they are waiting on upstream dependency compat fixes.

The downgrade CI workflows (`Downgrade.yml` and `DowngradeSublibraries.yml`) are currently set to `if: false` to skip them.

Once the upstream compat bounds are fixed, these tests should be re-enabled by removing the `if: false` lines.