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

Author: http4s-steward[bot]

project/plugins.sbt

@@ -1,4 +1,4 @@
 addSbtPlugin("org.http4s" % "sbt-http4s-org" % "0.16.2")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.14.0")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.15.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.4.17")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")