Update sbt-scalafmt to 2.5.6

Author: scala-steward

project/plugins.sbt

@@ -1,4 +1,4 @@
 libraryDependencies += "org.scala-sbt" %% "scripted-plugin" % sbtVersion.value
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.13.1")
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")