Fix unreleased changelog "-30974-issue-search-by-number.yml"

What does this MR do?

Fix the merge_request value in the unreleased changelog yaml file,

Online YAML Parser validate result here:

• https://yaml-online-parser.appspot.com/?url=https%3A%2F%2Fgitlab.com%2Fgitlab-org%2Fgitlab-ce%2Fraw%2Fb33fb07ef9d2fdf0ce983ded13f9c36ed02e2fd8%2Fchangelogs%2Funreleased%2F-30974-issue-search-by-number.yml

I also removed the leading slash in the filename to prevent some unfriendly command line behavior.

Please take a look at the commit message for more details, thanks.

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I removed other parts of the merge request template as I think they are not related in this case. I also separated these two changes into two different commit as I think they are both meaningful individually, please don't squash them if possible.

cc !28302 (merged)