Convert Pivotaltracker to use api_arguments

What does this MR do and why?

We're converting IntegrationPivotalTracker to use api_arguments.

References

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MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Checkout the branch
2. Verify that IntegrationPivotalTracker API works as expected 🎉

Example query

curl --request PUT \
  --url http://172.16.123.1:3000/api/v4/projects/<PROJECT_ID>/integrations/pivotaltracker \
  --header 'PRIVATE-TOKEN: <PRIVATE_TOKEN>' \
  --data '{
	"token": "123456",
	"restrict_to_branch": "master,main"
}'