Jira app - improve setup required instructions for GitLab.com

What does this MR do and why?

Based on the #430044 (closed), we're updating instructions for GitLab.com installations.

Screenshots or screen recordings

Before	After

How to set up and validate locally

suggestion: Use the https://gitlab.com/gitlab-org/manage/integrations/jira-connect-test-tool to validate locally.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430044 (closed)