Roll Ubuntu to ubuntu-latest

Author: DavisVaughan

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

@@ -31,13 +31,13 @@ jobs:
           # - {os: windows-latest, r: '3.6'}
 
           # Use older ubuntu to maximise backward compatibility
-          - {os: ubuntu-18.04,   r: 'devel', http-user-agent: 'release'}
-          - {os: ubuntu-18.04,   r: 'release'}
-          - {os: ubuntu-18.04,   r: 'oldrel-1'}
-          - {os: ubuntu-18.04,   r: 'oldrel-2'}
-          - {os: ubuntu-18.04,   r: 'oldrel-3'}
+          - {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'}
           # Don't use R 3.4 build. survival, mda, TH.data require >=3.5.
-          # - {os: ubuntu-18.04,   r: 'oldrel-4'}
+          # - {os: ubuntu-latest,   r: 'oldrel-4'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}