Fix CI deprecation warnings in GitHub Actions workflows

- Replace julia-actions/julia-buildpkg@latest with @v1.7.0 in Downstream.yml
- Update julia-actions/julia-buildpkg from @v1 to @v1.7.0 (latest) in Downgrade.yml
- Update julia-actions/julia-runtest from @v1 to @v1.11.2 (latest) in Downgrade.yml

These changes address deprecated @latest tag usage and update actions to their most recent stable versions.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: claude

.github/workflows/Downgrade.yml

@@ -28,8 +28,8 @@ jobs:
 #        if: ${{ matrix.version == '1.6' }}
         with:
           skip: Pkg,TOML
-      - uses: julia-actions/julia-buildpkg@v1
-      - uses: julia-actions/julia-runtest@v1
+      - uses: julia-actions/julia-buildpkg@v1.7.0
+      - uses: julia-actions/julia-runtest@v1.11.2
         with:
           ALLOW_RERESOLVE: false
         env:

.github/workflows/Downstream.yml

@@ -28,7 +28,7 @@ jobs:
         with:
           version: ${{ matrix.julia-version }}
           arch: x64
-      - uses: julia-actions/julia-buildpkg@latest
+      - uses: julia-actions/julia-buildpkg@v1.7.0
       - name: Clone Downstream
         uses: actions/checkout@v4
         with: