Update sbt-scalafmt to 2.5.6

Author: scala-steward

project/plugins.sbt

@@ -1,3 +1,3 @@
 addSbtPlugin("com.github.sbt" % "sbt-ci-release"     % "1.11.2")
-addSbtPlugin("org.scalameta"  % "sbt-scalafmt"       % "2.5.5")
+addSbtPlugin("org.scalameta"  % "sbt-scalafmt"       % "2.5.6")
 addSbtPlugin("com.github.sbt" % "sbt-github-actions" % "0.28.0")