[ObjC] Add a C Preprocessor Check for removing some code in the future

PiperOrigin-RevId: 740886561

Author: thomasvl

objectivec/GPBDescriptor.m

@@ -153,9 +153,13 @@ + (instancetype)allocDescriptorForClass:(Class)messageClass
   GPBFieldFlags unknownFieldFlags = (GPBFieldFlags)(~(
       GPBFieldRequired | GPBFieldRepeated | GPBFieldPacked | GPBFieldHasDefaultValue |
       GPBFieldClearHasIvarOnZero | GPBFieldTextFormatNameCustom | GPBFieldMapKeyMask));
+#if GOOGLE_PROTOBUF_OBJC_MIN_SUPPORTED_VERSION > 40310
+#error "Time to remove these methods"
+#else
   if (runtimeSupport == &GOOGLE_PROTOBUF_OBJC_EXPECTED_GENCODE_VERSION_40310) {
     unknownFieldFlags &= ~GPBFieldOptional;  // Allow 40310 to still have GPBFieldOptional.
   }
+#endif
   NSAssert((mergedFieldFlags & unknownFieldFlags) == 0, @"Internal error: unknown field flags set");
 #pragma clang diagnostic pop
 #endif  // defined(DEBUG) && DEBUG