ci(github): simplify R-CMD-check to just run on ubunutu latest (and not also the development version and old versions)

Author: amyheather

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

@@ -21,9 +21,7 @@ jobs:
         config:
           - {os: macos-latest,   r: 'release'}
           - {os: windows-latest, r: 'release'}
-          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
-          - {os: ubuntu-latest,   r: 'oldrel-1'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}