Replace ubuntu-20.04 runner

https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

Author: nsoranzo

.github/workflows/pr.yaml

@@ -188,7 +188,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-24.04]
         r-version: ['release']
     steps:
     - uses: actions/checkout@v4