Update scala-library to 2.12.20 (#374)

Author: scala-steward

build.sbt

@@ -2,8 +2,8 @@ import com.typesafe.tools.mima.core._
 
 inThisBuild(
   List(
-    scalaVersion := "2.12.19",
-    crossScalaVersions := Seq("2.12.19", "2.13.14"),
+    scalaVersion := "2.12.20",
+    crossScalaVersions := Seq("2.12.20", "2.13.14"),
     scalacOptions ++= List("-release", "8"),
     javacOptions ++= List("-target", "8", "-source", "8"),
     organization := "com.thesamet.scalapb"