Update sbt-unidoc to 0.6.0 in 0.11.x #2821

Job	Run time
conformance	1m 56s
build (2_12, e2e)	3m 13s
build (2_13, mima)	1m 26s
build (2_12, mima)	1m 29s
build (2_13, e2e)	4m 1s
build (2_13, test)	11m 54s
build (3, mima)	9s
build (2_12, test)	12m 43s
build (3, e2e)	4m 9s
build (2_13, scala3_compat_test)	8m 25s
build (3, test)	9m 56s
build (2_12, examples_and_formatting)	2m 59s
build (3, scala3_compat_test)	8m 57s
ci-passed	2s
	1h 11m 19s

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