8284858: Start of release updates for JDK 20

8286035: Add source 20 and target 20 to javac
8286034: Add SourceVersion.RELEASE_20

Reviewed-by: dholmes, kcr, iris, erikj, jjg, ihse

test/langtools/tools/javac/classfiles/ClassVersionChecker.java

@@ -54,7 +54,8 @@ public class ClassVersionChecker {
         SIXTEEN("16", 60),
         SEVENTEEN("17", 61),
         EIGHTEEN("18", 62),
-        NINETEEN("19", 63);
+        NINETEEN("19", 63),
+        TWENTY("20", 64);
 
         private Version(String release, int classFileVer) {
             this.release = release;