Update specs2-scalacheck to 5.6.3 in series/2.x

Author: typelevel-steward[bot]

build.sbt

@@ -16,7 +16,7 @@ ThisBuild / licenses := Seq("MIT" -> url("https://opensource.org/licenses/MIT"))
 ThisBuild / startYear := Some(2019)
 
 val disciplineV = "1.7.0"
-val specs2V = "5.5.8"
+val specs2V = "5.6.3"
 
 lazy val `discipline-specs2` =
   tlCrossRootProject.aggregate(core).settings(name := "discipline-specs2")