Update scala-library to 2.12.20 in 0.12.x (#535)

Author: scala-steward

build.sbt

@@ -2,7 +2,7 @@ import scalapb.compiler.Version.scalapbVersion
 
 scalaVersion := "2.13.12"
 
-crossScalaVersions := Seq("2.12.18", "2.13.16", "3.3.3")
+crossScalaVersions := Seq("2.12.20", "2.13.16", "3.3.3")
 
 ThisBuild / organization := "com.thesamet.scalapb"