Update sbt-mdoc to 2.6.3 #450

Job	Run time
Mima binary compatibility test	1m 11s
scalajs-1.5.1 scala-2.12.14 jdk-8 tests	1m 44s
scalajs-1.5.1 scala-2.13.6 jdk-8 tests	1m 41s
scala-2.12.14 jdk-8 tests	1m 50s
scala-2.13.6 jdk-8 tests	1m 53s
scalajs-1.5.1 scala-3.0.0 jdk-8 tests	1m 40s
scala-3.0.0 jdk-8 tests	1m 50s
scalajs-1.5.1 scala-2.12.14 jdk-11 tests	2m 4s
scalajs-1.5.1 scala-2.13.6 jdk-11 tests	1m 59s
scalajs-1.5.1 scala-3.0.0 jdk-11 tests	1m 59s
scala-2.12.14 jdk-11 tests	2m 12s
scala-2.13.6 jdk-11 tests	2m 14s
scala-3.0.0 jdk-11 tests	2m 9s
publish	0s
	24m 26s

Provide feedback