Update sbt-mdoc to 2.6.4 #2426

Job	Run time
conformance	4m 15s
build (2_12, e2e)	3m 34s
build (2_12, test)	13m 1s
build (2_13, e2e)	4m 25s
build (2_13, test)	11m 12s
build (3, e2e)	4m 58s
build (3, test)	8m 49s
build (2_12, examples_and_formatting)	3m 9s
build (2_13, scala3_compat_test)	8m 3s
build (3, scala3_compat_test)	10m 56s
ci-passed	1s
	1h 12m 23s

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