Update logback-classic to 1.5.15

Author: gu-scala-steward-public-repos[bot]

build.sbt

@@ -38,7 +38,7 @@ lazy val root = (project in file("."))
       ++ Seq(
         "net.logstash.logback" % "logstash-logback-encoder" % "8.0",
         // Transient dependency of Play. No newer version of Play 2.9 or Play 3.0 with this vulnerability fixed.
-        "ch.qos.logback" % "logback-classic" % "1.5.13",
+        "ch.qos.logback" % "logback-classic" % "1.5.15",
       ),
 
     buildInfoKeys ++= Seq[BuildInfoKey](