Update scala-compiler, scala-library, ... to 2.12.21

Author: typelevel-steward[bot]

build.sbt

@@ -8,7 +8,7 @@ ThisBuild / organization := "org.typelevel"
 ThisBuild / organizationName := "Typelevel"
 ThisBuild / startYear := Some(2023)
 
-ThisBuild / crossScalaVersions := Seq("3.3.7", "2.12.20", "2.13.18")
+ThisBuild / crossScalaVersions := Seq("3.3.7", "2.12.21", "2.13.18")
 ThisBuild / scalaVersion := (ThisBuild / crossScalaVersions).value.head
 
 ThisBuild / doctestTestFramework := DoctestTestFramework.ScalaCheck