Update GHA to use latest Ubuntu

Author: IndrajeetPatil

.github/workflows/check-full.yaml

@@ -27,12 +27,12 @@ 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-18.04,   r: 'oldrel-4'}
+          - {os: ubuntu-20.04,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-20.04,   r: 'release'}
+          - {os: ubuntu-20.04,   r: 'oldrel-1'}
+          - {os: ubuntu-20.04,   r: 'oldrel-2'}
+          - {os: ubuntu-20.04,   r: 'oldrel-3'}
+          - {os: ubuntu-20.04,   r: 'oldrel-4'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/pre-commit.yaml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   pre-commit:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     if: >-
       !contains(github.event.head_commit.message, 'ci skip') &&
       (

cran-comments.md

@@ -10,7 +10,7 @@ Luke Tierney.
 
 ## Test environments
 
--   ubuntu 18.04 (on GitHub Actions): R devel, R 4.1.2, R 4.0.5, R 3.6, R 3.5,
+-   ubuntu 20.04 (on GitHub Actions): R devel, R 4.1.2, R 4.0.5, R 3.6, R 3.5,
     R 3.4
 -   Windows Server 10 (on GitHub Actions): R 3.6, R 4.0.5
 -   win-builder: R devel