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 (closed).

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 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#24659 (closed)