Update scala-library to 2.12.21

http4s-steward[bot] · http4s-steward[bot] · commit 500c3a5ecc87 · 2025-12-12T01:10:17.000Z
diff --git a/build.sbt b/build.sbt
@@ -21,7 +21,7 @@ lazy val docs = project
   .enablePlugins(Http4sOrgSitePlugin)
 
 ThisBuild / tlBaseVersion := "2.0"
-ThisBuild / crossScalaVersions := Seq("2.12.20")
+ThisBuild / crossScalaVersions := Seq("2.12.21")
 ThisBuild / developers := List(
   Developer(
     "rossabaker",
