Draft: Use different endpoint to fetch user information for Jira Connect app

What does this MR do and why?

The user endpoint requires the "Browse users and groups" global permission [1], which some users disable for security reasons.

Instead we can use the myself endpoint, which doesn't require any special permissions [2] and should always be available for users who have permission to manage the Jira Connect app.

[1] https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-users/#api-rest-api-3-user-get
[2] https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-myself/#api-rest-api-3-myself-get

Issue: #351442

How to set up and validate locally

The "Browse users and groups" permission can be managed on https://yourjirainstance.atlassian.net/secure/admin/GlobalPermissions!default.jspa, but I wasn't able to reproduce the problem with the user endpoint after removing all of these groups (even after waiting 15+ minutes, in case there's a delay with permission changes):

But we really just need to make sure that the myself endpoint works correctly too:

1. Open this MR in Gitpod and make port 3000 public.
2. Install the app from the Gitpod instance: https://docs.gitlab.com/ee/integration/jira/connect-app.html#install-the-application-manually
3. Link a namespace as described on https://docs.gitlab.com/ee/integration/jira/connect-app.html#install-the-gitlabcom-for-jira-cloud-app, the Jira user information is fetched when clicking Link on a namespace.
4. This should succeed without errors.

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 #351442