ci: use ubuntu-latest and actions/checkout@v4 in R-CMD-check

Author: gadenbuie

.github/workflows/R-CMD-check.yaml

@@ -28,20 +28,19 @@ jobs:
           # Use 3.6 to trigger usage of RTools35
           - {os: windows-latest, r: '3.6'}
 
-          # Use older ubuntu to maximise backward compatibility
-          - {os: ubuntu-20.04,   r: 'devel', http-user-agent: 'release'}
-          - {os: ubuntu-20.04,   r: 'release'}
-          - {os: ubuntu-20.04,   r: 'oldrel-1'}
-          - {os: ubuntu-20.04,   r: 'oldrel-2'}
-          - {os: ubuntu-20.04,   r: 'oldrel-3'}
-          - {os: ubuntu-20.04,   r: 'oldrel-4'}
+          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-latest,   r: 'release'}
+          - {os: ubuntu-latest,   r: 'oldrel-1'}
+          - {os: ubuntu-latest,   r: 'oldrel-2'}
+          - {os: ubuntu-latest,   r: 'oldrel-3'}
+          - {os: ubuntu-latest,   r: 'oldrel-4'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
       R_KEEP_PKG_SOURCE: yes
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - uses: r-lib/actions/setup-pandoc@v2