chore: version bump built_value and built_value_generator #935

Job	Run time
test / test-dart-vm (3.0.1)	1m 59s
test / test-dart-vm (stable)	1m 37s
dart2js_test / test-dart-2js (3.0.1, chrome)	3m 9s
ddc_test / test-dart-ddc (3.0.1, chrome)	2m 44s
native_test / test-dart-native (macos-latest)	2m 8s
dart2js_test / test-dart-2js (3.0.1, firefox)	3m 0s
ddc_test / test-dart-ddc (3.0.1, firefox)	2m 31s
native_test / test-dart-native (windows-latest)	3m 31s
dart2js_test / test-dart-2js (stable, chrome)	2m 57s
ddc_test / test-dart-ddc (stable, chrome)	2m 42s
dart2js_test / test-dart-2js (stable, firefox)	2m 49s
ddc_test / test-dart-ddc (stable, firefox)	2m 34s
	31m 41s

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