Update scala-library to 2.12.21

Author: scalikejdbc-bot[bot]

build.sbt

@@ -2,7 +2,7 @@ lazy val _version = "0.20.1-SNAPSHOT"
 lazy val scalikejdbcVersion = "4.3.5"
 lazy val jasyncVersion = "2.2.4" // provided
 lazy val postgresqlVersion = "42.7.8"
-val Scala212 = "2.12.20"
+val Scala212 = "2.12.21"
 val Scala213 = "2.13.18"
 val Scala3 = "3.3.7"