Stop CI on R 3.6 (#5888)

Author: yutannihilation

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

@@ -25,8 +25,6 @@ jobs:
           - {os: macos-latest,   r: 'release'}
 
           - {os: windows-latest, r: 'release'}
-          # Use 3.6 to trigger usage of RTools35
-          - {os: windows-latest, r: '3.6'}
           # use 4.1 to check with rtools40's older compiler
           - {os: windows-latest, r: '4.1'}