bump ubuntu version on GHA (#800)

Author: simonpcouch

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

@@ -26,10 +26,10 @@ jobs:
           - {os: macOS-latest,   r: 'release'}
           - {os: windows-latest, r: 'release'}
           # 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-latest,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-latest,   r: 'release'}
+          - {os: ubuntu-latest,   r: 'oldrel-1'}
+          - {os: ubuntu-latest,   r: 'oldrel-2'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/old-tensorflow.yaml

@@ -25,7 +25,7 @@ jobs:
         config:
           - {os: windows-latest, r: 'release'}
           # Use older ubuntu to maximise backward compatibility
-          - {os: ubuntu-18.04,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
       R_KEEP_PKG_SOURCE: yes