Merge branch '22592-can-set-due-date-through-slash-commands-even-though-i-m-not-authorized-to' into 'master'

Fix permission for setting an issue's due date

## What does this MR do?

This merge request ensure the current user can `:admin_issue` in order to change the issue's `due_date`, in `BaseIssuableService` and in `SlashCommands::InterpretService`.

Closes #22592 

## Are there points in the code the reviewer needs to double check?

No.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] 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 !6539