Flip --experimental_worker_allow_json_protocol

Based on the discussion here
bazelbuild#13607 we want to flip this and
leave it around temporarily, but there are also some refactorings coming
around how to do that. This flips the value now so it can be used
without the flag sooner in rolling releases.

src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java

@@ -48,7 +48,7 @@ public class WorkerOptions extends OptionsBase {
 
   @Option(
       name = "experimental_worker_allow_json_protocol",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
       effectTags = {OptionEffectTag.BUILD_FILE_SEMANTICS},
       help =