Update sbt-header to 5.6.0

Author: scala-steward

project/plugins.sbt

@@ -1,4 +1,4 @@
 addSbtPlugin("com.dwijnand"      % "sbt-dynver"   % "4.0.0")
 addSbtPlugin("org.foundweekends" % "sbt-bintray"  % "0.5.6")
-addSbtPlugin("de.heikoseeberger" % "sbt-header"   % "5.5.0")
+addSbtPlugin("de.heikoseeberger" % "sbt-header"   % "5.6.0")
 addSbtPlugin("org.scalameta"     % "sbt-scalafmt" % "2.3.4")