[BEAM-7179] Correct file extension

[AaronLeeIV]

DATAFLOW_GRAPH_FILE_NAME (String DATAFLOW_GRAPH_FILE_NAME = "dataflow_graph.pb") 's file extension is set to .pb however it is indeed a .json.

The content is generated as

options
  .getStager()
  .stageToFile(
      DataflowPipelineTranslator.jobToString(newJob).getBytes(UTF_8),
      DATAFLOW_GRAPH_FILE_NAME);

see also DataflowPipelineTranslator

While the actual proto String PIPELINE_FILE_NAME = "pipeline.pb"
is generated as

options.getStager().stageToFile(serializedProtoPipeline, PIPELINE_FILE_NAME);

---

@lukecwik Can you take a look.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[lukecwik]

Could you reference a JIRA (file one if it doesn't exist) related to this change?

[AaronLeeIV]

Indeed, filed issue and updated the subject.

[AaronLeeIV]

retest this please

[AaronLeeIV]

retest this please

[AaronLeeIV]

retest this please

[lukecwik]

Run Java PreCommit

[lukecwik]

Run Java PreCommit

[lukecwik]

Run Java PreCommit