Bump gradle/actions from 5 to 6 (#421) #321

Job	Run time
build-and-test / check-style	3m 44s
build-and-test / check-windows / check	5m 12s
build-and-test / check-macos / check	5m 38s
build-and-test / check-linux / check	7m 17s
publish	21m 0s
	42m 51s

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