Simplify copy on "Create a new list" dropdown in Issue Boards

## What does this MR do?

It changes the "Create new list" text to simply "Add list". It also describes the operation in a more concise way. It was suggested in #24659.

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

Although I refactored references to the name of the button to reflect the new text, I left part of the documentation unmodified:

File: `doc/user/project/issue_board.md`
```
In short, here's a list of actions you can take in an Issue Board:                 
                                                                                   
- [Create a new list](#creating-a-new-list).                                       
- [Delete an existing list](#deleting-a-list). 
```
because I was not sure if changing those was necessary.


Additionally, I replaced every occurrence of `click_button 'Create new list'` with `click_button 'Add list'`in the spec. I'm not sure if that causes any change in behavior as I am not sure if there are any hooks to the previous name, explicitly.

File: `spec/features/boards/boards_spec.rb`

## Why was this MR needed?

It is an improvement, as suggested in #24659

## Screenshots (if relevant)

**Before:**
![before](/uploads/cffd30b0e9a21d16bc25d046eb4ca42b/before.png)

**After:**
![after2](/uploads/95b049e0c1c565c4ccd1a72a3f4be925/after2.png)

## Does this MR meet the acceptance criteria?

- [X] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [X] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#24659

See merge request !7605