chore(workflows): Use ubuntu-latest #650

Job	Run time
test / test-dart-vm (3.0.1)	1m 50s
test / test-dart-vm (stable)	1m 40s
native_test / test-dart-native (macos-latest)	2m 40s
native_test / test-dart-native (windows-latest)	4m 42s
dart2js_test / test-dart-2js (3.0.1, chrome)	8m 26s
dart2js_test / test-dart-2js (3.0.1, firefox)	8m 14s
dart2js_test / test-dart-2js (stable, chrome)	8m 14s
ddc_test / test-dart-ddc (3.0.1, chrome)	6m 6s
dart2js_test / test-dart-2js (stable, firefox)	7m 24s
ddc_test / test-dart-ddc (3.0.1, firefox)	6m 14s
ddc_test / test-dart-ddc (stable, chrome)	5m 54s
ddc_test / test-dart-ddc (stable, firefox)	5m 17s
	1h 6m 41s

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