JIRA Integration: Fix API URL with trailing slash

What does this MR do?

This merge request fixes the JIRA Integration using the API URL with a trailing slash.

The JIRA integration already works with the URL with a trailing slash.

Related to #14945 (closed) and #14946 (closed).

Previous related merge requests:

• https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/3266/
• gitlab-foss!30911 (merged)
• gitlab-foss!16748 (merged)