update GH ubuntu versions

Author: bbolker

.github/workflows/check-standard.yaml

@@ -24,8 +24,8 @@ jobs:
         config:
           - {os: windows-latest, r: 'release'}
           - {os: macOS-latest, r: 'release'}
-          - {os: ubuntu-20.04, r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/focal/latest"}
-          - {os: ubuntu-20.04, r: 'devel', rspm: "https://packagemanager.rstudio.com/cran/__linux__/focal/latest"}
+          - {os: ubuntu-latest, r: 'release'}
+          - {os: ubuntu-latest, r: 'devel'}
 
     env:
       R_REMOTES_NO_ERRORS_FROM_WARNINGS: true