[ObjC] mark mergeFromData:extensionRegistry: as deprecated.

Since it really isn't safe in Swift, point folks at the new api that uses errors.

PiperOrigin-RevId: 511262228

objectivec/GPBMessage.h

@@ -276,7 +276,9 @@ CF_EXTERN_C_END
  *                                         unsuccessful.
  **/
 - (void)mergeFromData:(NSData *)data
-    extensionRegistry:(nullable id<GPBExtensionRegistry>)extensionRegistry;
+    extensionRegistry:(nullable id<GPBExtensionRegistry>)extensionRegistry
+    __attribute__((deprecated(
+        "Use -mergeFromData:extensionRegistry:error: instead, especaily if calling from Swift.")));
 
 /**
  * Parses the given data as this message's class, and merges those values into

objectivec/GPBMessage.m

@@ -3261,7 +3261,10 @@ static void ResolveIvarSet(__unsafe_unretained GPBFieldDescriptor *field,
   if (self) {
     NSData *data = [aDecoder decodeObjectOfClass:[NSData class] forKey:kGPBDataCoderKey];
     if (data.length) {
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
       [self mergeFromData:data extensionRegistry:nil];
+#pragma clang diagnostic pop
     }
   }
   return self;

objectivec/Tests/GPBMessageTests.m

@@ -224,7 +224,11 @@
   result = [self mergeExtensionsDestination];
   NSData *data = [[self mergeExtensionsSource] data];
   XCTAssertNotNil(data);
-  [result mergeFromData:data extensionRegistry:[UnittestRoot extensionRegistry]];
+  NSError *error = nil;
+  XCTAssertTrue([result mergeFromData:data
+                    extensionRegistry:[UnittestRoot extensionRegistry]
+                                error:&error]);
+  XCTAssertNil(error);
   resultData = [result data];
   XCTAssertEqualObjects(resultData, mergeResultData);
   XCTAssertEqualObjects(result, [self mergeExtensionsResult]);