Update sbt-scalafmt to 2.6.1 (#940)

Co-authored-by: typelevel-steward[bot] <106827141+typelevel-steward[bot]@users.noreply.github.com>

Author: typelevel-steward[bot]

project/plugins.sbt

@@ -6,6 +6,6 @@ addSbtPlugin("org.typelevel" % "sbt-typelevel-site" % sbtTypelevelVersion)
 
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.4.4")
 
-addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.6")
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.6.1")
 
 addSbtPlugin("com.timushev.sbt" % "sbt-updates" % "0.6.4")