chore: moved user agent to logging category api #1895

Job	Run time
test / test-dart-vm (3.0.1)	2m 20s
test / test-dart-vm (stable)	2m 14s
dart2js_test / test-dart-2js (3.0.1, chrome)	3m 49s
dart2js_test / test-dart-2js (3.0.1, firefox)	3m 59s
ddc_test / test-dart-ddc (3.0.1, chrome)	3m 48s
native_test / test-dart-native (macos-latest)	5m 2s
dart2js_test / test-dart-2js (stable, chrome)	3m 50s
native_test / test-dart-native (windows-latest)	2m 51s
dart2js_test / test-dart-2js (stable, firefox)	3m 45s
ddc_test / test-dart-ddc (3.0.1, firefox)	3m 19s
ddc_test / test-dart-ddc (stable, chrome)	4m 43s
ddc_test / test-dart-ddc (stable, firefox)	3m 9s
	42m 49s

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