Update scala-library to 2.12.21

Author: typelevel-steward[bot]

build.sbt

@@ -15,7 +15,7 @@ ThisBuild / semanticdbVersion := scalafixSemanticdb.revision
 
 ThisBuild / versionScheme := Some(VersionScheme.EarlySemVer)
 
-ThisBuild / scalaVersion := "2.12.20"
+ThisBuild / scalaVersion := "2.12.21"
 
 lazy val `sbt-tpolecat` = project
   .in(file("."))