Merge pull request #2870 from github/mbg/ci/retire-ubuntu-20.04 #4096

Job	Run time
Diagnostic export (macos-latest, linked)	1m 21s
Diagnostic export (ubuntu-latest, linked)	1m 5s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 2s
Diagnostic export (windows-latest, linked)	5m 34s
Diagnostic export (windows-latest, nightly-latest)	4m 30s
Diagnostic export (macos-latest, nightly-latest)	1m 2s
	14m 34s

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