Chrome and other browsers ignore `autocomplete=false` and
`autocomplete=off` now. This likely caused the mirroring section to
expand whenever a user clicked "Expand" in the protected branches
settings.

https://developers.google.com/web/fundamentals/design-and-ux/input/forms/?hl=en
recommends using `new-password` as the new field to prevent automatic
filling.

This is similar to
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8477.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11237