Self-managed support extended to GitLab for Jira app
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### Background
In https://gitlab.com/gitlab-org/gitlab/-/issues/321401 we did investigation and created a [prototype](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54378) for a proxy of the Atlassian connect app that could be used to configure self-managed GitLab instances. This epic is to take that work and complete the proxy.
### Problem to solve
Users will have clear instruction when setting up the app and connecting namespaces via the Atlassian Marketplace, along with a fully fledged OAuth workflow. This will also open up the path for deprecating the DVCS connector and migrating Self-managed users to the Jira Cloud App.
### Release notes
For self-managed GitLab users, we're excited to announce support for the [GitLab for Jira App](https://marketplace.atlassian.com/apps/1221011/gitlab-com-for-jira-cloud?tab=overview&hosting=cloud)! With this release, users will be able to link their self-managed GitLab instance to Jira Cloud and enable the [Jira Development Panel](https://docs.gitlab.com/ee/integration/jira/development_panel.html). This feature supports syncing updates from smart commits, merge requests, branches, builds, deploys and feature flags back into the Jira development panel.
This integration can be paired with [GitLab's Basic Jira Integration](https://docs.gitlab.com/ee/integration/jira/configure.html) to also enable Jira issues in GitLab and take advantage of additional web links and comment links.



Learn more about how to get started with the[ GitLab for Jira App with your self-managed GitLab instance here](https://docs.gitlab.com/ee/integration/jira/connect-app.html).
_Note: for Jira Server or Jira DataCenter users, we recommend using the [Jira DVCS Connector](https://docs.gitlab.com/ee/integration/jira/dvcs.html)._
## UI Updates
| Step 1 - Choose GitLab version | Step 2 - Link namespaces |
| ------ | ------ |
|  |  |
### Proposal walkthrough
- :tv: https://youtu.be/Bg1yZt3UOV0
### Design source
- :pencil2: [Figma project](https://www.figma.com/file/FiMwdMsOL4UCtUzIGPrXXD/Create-proxy-for-self-managed-GitLab-%265650?node-id=306%3A22)
epic