chore: Test with Protobuf-Java v4.29.4

Author: lqiu96

.kokoro/build.sh

@@ -62,6 +62,7 @@ javadoc)
 integration)
     mvn -B ${INTEGRATION_TEST_ARGS} \
       -ntp \
+      -Dprotobuf.version=4.29.4 \
       -Penable-integration-tests \
       -DtrimStackTrace=false \
       -Dclirr.skip=true \

README.md

@@ -43,7 +43,7 @@ If you are using Maven without the BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-pubsub</artifactId>
-  <version>1.140.2</version>
+  <version>1.141.0</version>
 </dependency>
 
 ```