Upgrade logback-classic from 1.5.20 to 1.5.21 (waylayio#265)

Author: scala-steward

build.sbt

@@ -4,7 +4,7 @@ val playJsonVersion      = "3.0.6"
 val playVersion          = "2.7.3" // test only
 val playWsVersion        = "2.1.11"
 val slf4jVersion         = "2.0.17"
-val logbackVersion       = "1.5.20"
+val logbackVersion       = "1.5.21"
 val specs2Version        = "4.23.0"
 val dockerTestkitVersion = "0.12.0"