Refine copy for Jira integration
What does this MR do and why?
Mostly copy changes as suggested by Technical Writing in #332280[jira-proposal-project-level.png]
![#332280[jira-proposal-project-level.png]](https://gitlab.com/gitlab-org/gitlab/-/issues/332280/designs/jira-proposal-project-level.png){kind=link}
Changelog: changed
Screenshots or screen recordings
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How to set up and validate locally
Go to Project > Settings > Integrations, select Jira. The URL should look like http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/integrations/jira/edit.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #332280 (closed)