Drop R 4.0 tests (#390)

Since magick doesn't work with it. Not a big deal since R 4.5 will be coming out soon and we'll be switching to R 4.1 anyway.

Author: hadley

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

@@ -24,18 +24,16 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - {os: macos-latest,   r: 'release'}
-
-          - {os: windows-latest, r: 'release'}
-          # use 4.0 or 4.1 to check with rtools40's older compiler
-          - {os: windows-latest, 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'}
+          - { 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" }
+          - { os: ubuntu-latest, r: "oldrel-2" }
+          - { os: ubuntu-latest, r: "oldrel-3" }
+          #- {os: ubuntu-latest,  r: 'oldrel-4'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}