Merge pull request #2354 from scala-steward/update/scala-library-2.12.21

Update scala-library, scala-reflect to 2.12.21

Author: jatcwang

build.sbt

@@ -22,7 +22,7 @@ lazy val munitVersion = "1.2.1"
 lazy val shapelessVersion = "2.3.13"
 lazy val silencerVersion = "1.7.1"
 lazy val specs2Version = "4.23.0"
-lazy val scala212Version = "2.12.20"
+lazy val scala212Version = "2.12.21"
 lazy val scala213Version = "2.13.18"
 lazy val scala3Version = "3.3.7"
 // scala-steward:off