Merge pull request #3648 from typelevel/update/scala-library-2.12.21

Update scala-library to 2.12.21

Author: mpilquist

build.sbt

@@ -11,7 +11,7 @@ ThisBuild / startYear := Some(2013)
 val Scala213 = "2.13.18"
 
 ThisBuild / scalaVersion := Scala213
-ThisBuild / crossScalaVersions := Seq("2.12.20", Scala213, "3.3.7")
+ThisBuild / crossScalaVersions := Seq("2.12.21", Scala213, "3.3.7")
 ThisBuild / tlVersionIntroduced := Map("3" -> "3.0.3")
 
 ThisBuild / githubWorkflowOSes := Seq("ubuntu-latest")