ci: use ubuntu20 for cgroup compatibility #32

Job	Run time
format / prettier	7s
lint / ansible	14s
test (centos8, true, -s default) / molecule	4m 25s
test (ubuntu2004, false, -s default) / molecule	5m 15s
test (ubuntu2004, true, -s ubuntu-jre8) / molecule	2m 8s
test (debian10, false, -s default) / molecule	4m 6s
check	1s
	16m 16s

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