Fix tooltip text when Copy to cliboard is clicked

## What does this MR do?

Fixes #22022 to change tooltip text to "Copied!" when the "Copy to clipboard" button is clicked.

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

Line 29 in app/assets/javascripts/copy_to_clipboard.js

## Why was this MR needed?

There was a regression that prevents tooltip change on button click for Copy to clipboard.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] 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 you do - 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?

Closes #22022

See merge request !6294
Signed-off-by: Rémy Coutable <remy@rymai.me>