Update sbt-assembly to 2.3.1 (#266)

Author: scala-steward

project/plugins.sbt

@@ -1,3 +1,3 @@
-addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.0")
+addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "2.3.1")
 addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.10.0")
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")