fix: reject trailing / in jira urls instead of trimming (#12772)

<!--A few plain bullets saying what changed and why, for a reviewer
skimming it - not a wall of text, not a restatement of the diff, not
generated boilerplate.-->
#### Description

This is so that a round trip drift doesn't happen. This was caught for
incident io in its PR but we missed it out here.

<!--Anything reviewers should keep in mind while reviewing -->
#### Additional Information

Found as a part of round trip testing in v2 notification channels create
API

<!--Please delete paragraphs that you did not use before submitting.-->