chore(ci): fix Node.js 20 deprecation and harden workflows

- Opt into Node.js 24 runtime via FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true.
- Pin actions/checkout and actions/setup-dotnet to immutable commit SHAs.
- Disable checkout credential persistence for better security.

Co-authored-by: Jadhielv <24376900+Jadhielv@users.noreply.github.com>

Author: google-labs-jules[bot]

.github/workflows/dotnet.yml

@@ -18,9 +18,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      with:
+        persist-credentials: false
     - name: Setup .NET
-      uses: actions/setup-dotnet@v4
+      uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # v4.3.1
       with:
         dotnet-version: 10.0.100
     - name: Restore dependencies