[gh-actions] update ubuntu 16.04 to 18.04

actions/runner-images#3287

Author: hbaniecki

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

@@ -39,15 +39,12 @@ jobs:
           - {os: macOS-latest,   r: '4.1'}
           - {os: macOS-latest,   r: '4.0'}
           - {os: macOS-latest,   r: '3.6'}
-          # - {os: macOS-latest,   r: '3.5'}
           - {os: windows-latest, r: '4.1'}
           - {os: windows-latest, r: '4.0'}
           - {os: windows-latest, r: '3.6'}
-          # - {os: windows-latest, r: '3.5'}
-          - {os: ubuntu-16.04, r: '4.1', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
-          - {os: ubuntu-16.04, r: '4.0', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
-          - {os: ubuntu-16.04, r: '3.6', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
-          # - {os: ubuntu-16.04, r: '3.5', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
+          - {os: ubuntu-18.04, r: '4.1', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
+          - {os: ubuntu-18.04, r: '4.0', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
+          - {os: ubuntu-18.04, r: '3.6', rspm: "https://demo.rstudiopm.com/all/__linux__/xenial/latest"}
 
     env:
       R_REMOTES_NO_ERRORS_FROM_WARNINGS: true