Ensures R-check is made on latest ubuntu

Author: LudvigOlsen

.github/workflows/R-check.yaml

@@ -25,9 +25,9 @@ jobs:
           - {os: windows-latest, r: 'release'}
           - {os: windows-latest, r: 'oldrel'}
 
-          - {os: ubuntu-18.04,   r: 'devel', http-user-agent: 'release'}
-          - {os: ubuntu-18.04,   r: 'release'}
-          - {os: ubuntu-18.04,   r: 'oldrel'}
+          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-latest,   r: 'release'}
+          - {os: ubuntu-latest,   r: 'oldrel'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}