Update sbt-scalajs, scalajs-compiler, ... to 1.15.0 #416

Job	Run time
Mima binary compatibility test	1m 13s
scala-2.12.14 jdk-8 tests	1m 52s
scalajs-1.5.1 scala-2.12.14 jdk-8 tests	1m 44s
scala-2.13.6 jdk-8 tests	1m 51s
scalajs-1.5.1 scala-2.13.6 jdk-8 tests	1m 46s
scala-3.0.0 jdk-8 tests	1m 54s
scalajs-1.5.1 scala-3.0.0 jdk-8 tests	1m 45s
scala-2.12.14 jdk-11 tests	2m 18s
scalajs-1.5.1 scala-2.12.14 jdk-11 tests	2m 6s
scala-2.13.6 jdk-11 tests	2m 17s
scalajs-1.5.1 scala-2.13.6 jdk-11 tests	1m 57s
scala-3.0.0 jdk-11 tests	2m 5s
scalajs-1.5.1 scala-3.0.0 jdk-11 tests	2m 9s
publish	0s
	24m 57s

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