Fix CI deprecation warnings

- Update julia-actions/setup-julia from v1 to v2 in Downstream.yml and ReleaseTest.yml
- Update julia-actions/julia-buildpkg and julia-actions/julia-runtest from v1 to @latest in Downgrade.yml
- Resolves Node.js 16 deprecation warnings by migrating to actions that support Node.js 20

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

Co-Authored-By: Claude <[email protected]>

Author: claude

.github/workflows/Downgrade.yml

@@ -28,8 +28,8 @@ jobs:
       - uses: julia-actions/julia-downgrade-compat@v2
         with:
           skip: Pkg,TOML
-      - uses: julia-actions/julia-buildpkg@v1
-      - uses: julia-actions/julia-runtest@v1
+      - uses: julia-actions/julia-buildpkg@latest
+      - uses: julia-actions/julia-runtest@latest
         with:
           ALLOW_RERESOLVE: false
         env:

.github/workflows/Downstream.yml

@@ -45,7 +45,7 @@ jobs:
           - {user: Neuroblox, repo: Neuroblox.jl, group: All}
     steps:
       - uses: actions/checkout@v4
-      - uses: julia-actions/setup-julia@v1
+      - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.julia-version }}
           arch: x64

.github/workflows/ReleaseTest.yml

@@ -29,7 +29,7 @@ jobs:
           - {package: Catalyst, group: All}
     steps:
       - uses: actions/checkout@v4
-      - uses: julia-actions/setup-julia@v1
+      - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.julia-version }}
           arch: x64