Bump required R version to 4.0 #1768

# to view logs

Job	Run time
R-CMD-check / macOS-latest (release)	5m 1s
R-CMD-check / windows-latest (devel)	6m 45s
R-CMD-check / windows-latest (release)	7m 31s
R-CMD-check / windows-latest (oldrel)	11m 22s
R-CMD-check / windows-latest (4.1)	8m 9s
R-CMD-check / ubuntu-latest (devel)	53m 57s
R-CMD-check / ubuntu-latest (release)	1m 59s
R-CMD-check / ubuntu-latest (4.3)	9m 42s
R-CMD-check / ubuntu-latest (4.2)	1m 8s
R-CMD-check / ubuntu-latest (4.1)	57s
R-CMD-check / ubuntu-latest (4.0)	52s
	1h 47m 23s

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