Fixed gtihub actions to use latest Ubuntu

Author: Roy Mendelssohn

.github/workflows/R-check.yml

@@ -15,7 +15,7 @@ jobs:
         - { os: windows-latest, r: 'release'}
         - { os: macOS-latest, r: 'release'}
         # - { os: macOS-latest, r: 'devel'}
-        - { os: ubuntu-18.04, r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/bionic/latest"}
+        - { os: ubuntu-latest, r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/bionic/latest"}
 
     env:
       R_REMOTES_NO_ERRORS_FROM_WARNINGS: true