Update sbt-mdoc to 2.8.0 in 0.11.x (#1991) #2910

Job	Run time
conformance	1m 54s
build (2_12, e2e)	3m 29s
build (2_13, test)	9m 48s
build (3, mima)	12s
build (2_13, mima)	1m 49s
build (2_12, examples_and_formatting)	3m 0s
build (2_12, test)	12m 26s
build (3, e2e)	4m 34s
build (2_12, mima)	1m 33s
build (2_13, e2e)	4m 24s
build (3, test)	7m 57s
build (2_13, scala3_compat_test)	9m 35s
build (3, scala3_compat_test)	7m 14s
ci-passed	2s
	1h 7m 57s

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