Adjust self-hosted Jira development panel integration

What does this MR do?

Returns 200 to the early requested API when integration with Jira development panel /api/v3/repos/-/jira/events.

Github enterprise returns internal events on this API (see https://developer.github.com/v3/activity/events/). It's not clear for now how exactly Jira uses it. Two options of what happened:

1. We've never reached this bug before, because they just make this request on self-hosted Jira instances
2. They've changed older versions with this extra request

Important to bring up that this endpoint is not requested on Jira Cloud. This worries me a bit regarding having to support multiple Jira instance versions.

It was tested on self-hosted Jira version 7.11.1 (Linux x64).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6811