Update ubuntu.yml

Author: MrJGao

.github/workflows/ubuntu.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        r-version: ['3.6.3', '4.2.1']
+        r-version: ['4.2.1']
 
     # Steps represent a sequence of tasks that will be executed as part of the
     # job