Mark com.google.protobuf.Protobuf.registerSchema as private

This is only used from within the Protobuf class in practice.

I am narrowing down this class in preparation to add some optimisations.

PiperOrigin-RevId: 816028146

Author: mhansen

java/core/src/main/java/com/google/protobuf/Protobuf.java

@@ -90,7 +90,7 @@ <T> Schema<T> schemaFor(T message) {
    * @return the previously registered schema, or {@code null} if the given schema was successfully
    *     registered.
    */
-  Schema<?> registerSchema(Class<?> messageType, Schema<?> schema) {
+  private Schema<?> registerSchema(Class<?> messageType, Schema<?> schema) {
     checkNotNull(messageType, "messageType");
     checkNotNull(schema, "schema");
     return schemaCache.putIfAbsent(messageType, schema);