Disable downgrade CI and skip pre tests due to JET incompatibility

- Add `if: false` to Downgrade.yml to skip downgrade tests (see issue #522)
- Exclude Core and Downstream groups from pre Julia since JET test
  dependency cannot be resolved on pre-release Julia versions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: claude

.github/workflows/Downgrade.yml

@@ -12,6 +12,7 @@ on:
       - 'docs/**'
 jobs:
   test:
+    if: false  # Disabled: JET test dependency incompatible with downgrade. See issue #522 for details.
     runs-on: ubuntu-latest
     strategy:
       matrix:

.github/workflows/Tests.yml

@@ -31,6 +31,11 @@ jobs:
           - "Downstream"
           - "nopre"
         exclude:
+          # JET is a test dependency that doesn't support pre-release Julia
+          - version: "pre"
+            group: "Core"
+          - version: "pre"
+            group: "Downstream"
           - version: "pre"
             group: "nopre"
     uses: "SciML/.github/.github/workflows/tests.yml@v1"