old R -> 3.6 (#2235)

Author: MichaelChirico

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

@@ -43,7 +43,7 @@ jobs:
           - {os: ubuntu-latest,  r: 'oldrel-2'}
           - {os: ubuntu-latest,  r: 'oldrel-3'}
           - {os: ubuntu-latest,  r: 'oldrel-4'}
-          - {os: ubuntu-latest,  r: '3.5'}
+          - {os: ubuntu-latest,  r: '3.6'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}