Update specs2-core, specs2-junit to 4.21.0 (waylayio#229)

Author: scala-steward

build.sbt

@@ -5,7 +5,7 @@ val playVersion          = "2.7.3" // test only
 val playWsVersion        = "2.1.11"
 val slf4jVersion         = "2.0.17"
 val logbackVersion       = "1.5.17"
-val specs2Version        = "4.20.9"
+val specs2Version        = "4.21.0"
 val dockerTestkitVersion = "0.12.0"
 
 val scala2_12 = "2.12.20"