Capitalize mentioned issue timeline notes
## What does this MR do?
Renames `mentioned in` to `Mentioned in` for issue timeline notes
## Are there points in the code the reviewer needs to double check?
Shouldn't be any 😄
## Why was this MR needed?
Resolves existing UI inconsistency as all the other issue timeline notes have their first letter capitalized.
## What are the relevant issue numbers?
Closes#21416
## Screenshots (if relevant)
Before:
After:
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
- [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
See merge request !6028
f2df4c689b