Update sbt-unidoc to 0.6.0 in 0.11.x (#1953) #2830

Job	Run time
conformance	1m 53s
build (2_13, test)	11m 0s
build (2_12, mima)	1m 21s
build (2_12, e2e)	3m 17s
build (2_12, test)	13m 33s
build (2_13, mima)	1m 29s
build (2_13, e2e)	4m 4s
build (3, mima)	7s
build (2_13, scala3_compat_test)	8m 6s
build (3, test)	10m 18s
build (3, scala3_compat_test)	6m 39s
build (2_12, examples_and_formatting)	2m 54s
build (3, e2e)	4m 15s
ci-passed	2s
	1h 8m 58s

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