fix: Replace deprecated protobuf methods. (#2764)

`isSynthetic()` is deprecated and will be
[removed](protocolbuffers/protobuf@1aeacd4#diff-2228551d02c6661809ca7103db9512eef4c2d01f35556d42316543d92a89edefL2846-L2847)
in protobuf-java 4.26.0, replace it so that we can prepare for
protobuf-java upgrade.

Per [official
doc](https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md)
of protobuf, an optional field is implemented as a "synthetic" oneof
field internally. Also see b/266950618#comment123 for suggested
replacements of `isSynthetic()`.

gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java

@@ -1063,12 +1063,10 @@ private static Field parseField(
         .setType(TypeParser.parseType(fieldDescriptor))
         .setIsMessage(fieldDescriptor.getJavaType() == FieldDescriptor.JavaType.MESSAGE)
         .setIsEnum(fieldDescriptor.getJavaType() == FieldDescriptor.JavaType.ENUM)
-        .setIsContainedInOneof(
-            fieldDescriptor.getContainingOneof() != null
-                && !fieldDescriptor.getContainingOneof().isSynthetic())
+        .setIsContainedInOneof(fieldDescriptor.getRealContainingOneof() != null)
         .setIsProto3Optional(
             fieldDescriptor.getContainingOneof() != null
-                && fieldDescriptor.getContainingOneof().isSynthetic())
+                && fieldDescriptor.getRealContainingOneof() == null)
         .setIsRepeated(fieldDescriptor.isRepeated())
         .setIsRequired(isRequired)
         .setFieldInfoFormat(fieldInfoFormat)