Update scala-library, scala-reflect to 2.12.21 (#550)

Author: softwaremill-ci

build.sbt

@@ -3,7 +3,7 @@ import com.softwaremill.SbtSoftwareMillCommon.commonSmlBuildSettings
 import com.softwaremill.Publish.ossPublishSettings
 import com.typesafe.tools.mima.core._
 
-val scala2_12 = "2.12.20"
+val scala2_12 = "2.12.21"
 val scala2_13 = "2.13.18"
 val scala2 = List(scala2_12, scala2_13)
 val scala2alive = List(scala2_12, scala2_13)