Update sbt-scoverage to 2.3.1 (#354)

Author: scala-steward

project/plugins.sbt

@@ -1,4 +1,4 @@
 addSbtPlugin("com.github.sbt"    % "sbt-ci-release" % "1.9.2")
 addSbtPlugin("de.heikoseeberger" % "sbt-header"     % "5.10.0")
-addSbtPlugin("org.scoverage"     % "sbt-scoverage"  % "2.3.0")
+addSbtPlugin("org.scoverage"     % "sbt-scoverage"  % "2.3.1")
 addSbtPlugin("org.scalameta"     % "sbt-scalafmt"   % "2.5.4")