Merge branch '20512-fix-rename-add-users-into-project-to-add-users-to-project-and-projects-ids-to-project-ids' into 'master'

Fix Rename `add_users_into_project` and `projects_ids`

## What does this MR do?
Only modifies the name of a method that leaves more semantic and expressive and the name of the keywords arguments to the rails convention.

## Are there points in the code the reviewer needs to double check?
Only if it has been changed at every point that is calling this method and that passing arguments.

## Why was this MR needed?
To make the code more expressive.

## What are the relevant issue numbers?
Closes #20512.

- [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 !5659