Use ubuntu-latest (#382)

Author: cderv

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

@@ -30,12 +30,12 @@ jobs:
           - {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-18.04,   r: 'oldrel-3'}
-          - {os: ubuntu-18.04,   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 }}

.github/workflows/required-latex-pkgs.yaml

@@ -14,7 +14,7 @@ name: check required LateX packages
 
 jobs:
   LateX-Packages:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     name: Ubuntu 18-04 (release)