chore(workflows): Use ubuntu-latest #2251

Job	Run time
test / test-dart-vm (3.0.1)	2m 49s
test / test-dart-vm (stable)	2m 46s
native_test / test-dart-native (macos-latest)	6m 44s
native_test / test-dart-native (windows-latest)	6m 39s
dart2js_test / test-dart-2js (3.0.1, chrome)	5m 19s
dart2js_test / test-dart-2js (3.0.1, firefox)	4m 39s
dart2js_test / test-dart-2js (stable, chrome)	4m 30s
dart2js_test / test-dart-2js (stable, firefox)	4m 36s
ddc_test / test-dart-ddc (3.0.1, chrome)	5m 18s
ddc_test / test-dart-ddc (3.0.1, firefox)	4m 49s
ddc_test / test-dart-ddc (stable, chrome)	4m 29s
ddc_test / test-dart-ddc (stable, firefox)	4m 19s
	56m 57s

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