Use Windows 3.6 instead of oldrel

Author: juliasilge

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

@@ -21,7 +21,7 @@ jobs:
           - {os: macOS-latest,   r: 'devel'}
           - {os: macOS-latest,   r: 'release'}
           - {os: windows-latest, r: 'release'}
-          - {os: windows-latest, r: 'oldrel'}
+          - {os: windows-latest, r: '3.6'}
           - {os: ubuntu-16.04,   r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/xenial/latest"}
           - {os: ubuntu-16.04,   r: 'oldrel',  rspm: "https://packagemanager.rstudio.com/cran/__linux__/xenial/latest"}
           - {os: ubuntu-16.04,   r: '3.5',     rspm: "https://packagemanager.rstudio.com/cran/__linux__/xenial/latest"}