Fix Jira integration steps

What does this MR do and why?

A customer raised a concern that the first steps in Configure a GitLab application for DVCS is not correct.

I have tested the steps and changed the wording to be concise. The user in GitLab will only be used to generate the secret and the client ID. Those credentials will then be used in Jira.

Zendesk ticket (Internal only)

Discussed the functionality in #350845 (closed)

Screenshots or screen recordings

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