Update scala-compiler, scala-library to 2.12.21

Author: dwolla-oss-scala-steward[bot]

build.sbt

@@ -1,7 +1,7 @@
 import org.typelevel.sbt.gha.MatrixExclude
 import org.typelevel.scalacoptions.ScalacOptions
 
-ThisBuild / crossScalaVersions := Seq("3.3.7", "2.13.18", "2.12.20")
+ThisBuild / crossScalaVersions := Seq("3.3.7", "2.13.18", "2.12.21")
 ThisBuild / scalaVersion := (ThisBuild / crossScalaVersions).value.head
 ThisBuild / githubWorkflowBuildMatrixExclusions := Seq(
   MatrixExclude(Map(