ci: re-enable downgrade job (strict) (#460)

Remove the `if: false` that was skipping the Downgrade workflow so it
runs again under the centralized downgrade.yml@v1 (strict,
allow_reresolve=false). The job is expected to be RED until the fntype
regression (#458/#452) is resolved, at which point it auto-greens.

Co-authored-by: Chris Rackauckas - Beep Boop Edition <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/Downgrade.yml

@@ -12,7 +12,7 @@ on:
       - 'docs/**'
 jobs:
   test:
-    if: false
+    # Runs strict (allow_reresolve=false); expected RED until fntype regression (#458/#452) is resolved.
     uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
     with:
       julia-version: "1.10"