Revert RoutingContext#user() returning UserContext to User. #1177

Job	Run time
IT / GraphQLWS-Tests (18.13.0)	50s
IT / SockJS-WriteHandler-Tests (16.13.1)	1m 20s
CI (ubuntu-latest, 11) / Run tests	6m 29s
CI (ubuntu-latest, 21) / Run tests	5m 55s
CI (windows-latest, 11) / Run tests	5m 50s
Deploy / Deploy to OSSRH	1m 23s
	21m 47s

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