Re-enable downgrade CI

The downgrade workflow was disabled. The julia-actions/julia-downgrade-compat
tooling issues that affected SciML monorepos are fixed and released in @v2, and
this package builds and passes its test suite against the downgraded (minimal
compatible) dependency versions. Re-enable the job and use allow_reresolve so
the test environment can resolve transitive/test-only dependencies the
downgrade step does not lock.

Verified locally on Julia 1.11: downgrade resolve + build + test all pass.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

.github/workflows/downgrade.yml

@@ -31,6 +31,6 @@ jobs:
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
         with:
-          ALLOW_RERESOLVE: false
+          allow_reresolve: true
         env:
           GROUP: ${{ matrix.group }}