Update scala-library to 2.12.21 in series/0.28

Author: http4s-steward[bot]

build.sbt

@@ -13,7 +13,7 @@ ThisBuild / startYear := Some(2014)
 ThisBuild / tlSitePublishBranch := Some("main")
 
 val Scala213 = "2.13.16"
-ThisBuild / crossScalaVersions := Seq(Scala213, "2.12.20", "3.3.7")
+ThisBuild / crossScalaVersions := Seq(Scala213, "2.12.21", "3.3.7")
 ThisBuild / scalaVersion := Scala213 // the default Scala
 ThisBuild / tlJdkRelease := Some(17)
 ThisBuild / githubWorkflowJavaVersions ~= {