GitLab for Jira app shows failed to load namespaces error

Summary

The GitLab.com for Jira Cloud app shows a "failed to load namespaces" error if the self-managed URL contains a trailing slash.

This bug came up in #386827 (comment 1233753178)

Steps to reproduce

1. Install the GitLab.com for Jira Cloud app.
2. Follow the guide to link your instance and enter your instance URL with a trailing slash.
3. See the error message

Workaround

1. Go back to the initial step where you enter your instance URL again. You might have to reload the page to see the "Change GitLab version" link.
2. Enter your instance URL and make sure there is no trailing slash.
3. Select Save again.
4. Log in again.