Update scala-library to 2.12.21 in series/0.24

Author: http4s-steward[bot]

build.sbt

@@ -11,7 +11,7 @@ ThisBuild / developers := List(
 ThisBuild / tlSitePublishBranch := Some("main")
 
 val Scala213 = "2.13.18"
-ThisBuild / crossScalaVersions := Seq("2.12.20", Scala213, "3.3.7")
+ThisBuild / crossScalaVersions := Seq("2.12.21", Scala213, "3.3.7")
 ThisBuild / scalaVersion := Scala213 // the default Scala
 
 // Undertow 2 for testing, requires Java 8 or higher