Update specs2-core, specs2-junit to 4.21.0 (#256)

Author: scala-steward

build.sbt

@@ -14,7 +14,7 @@ ThisBuild / developers := List(
 ThisBuild / licenses := List("MIT License" -> url("http://www.opensource.org/licenses/mit-license.php"))
 val playWsVersion   = "3.0.7"
 val playJsonVersion = "3.0.4"
-val specs2Version   = "4.20.9"
+val specs2Version   = "4.21.0"
 val pekkoVersion    = "1.0.3"
 
 val testContainersVersion = "0.43.0"