E2E test: wait for auto-merge to dissappear

What does this MR do and why?

In staging, tests that are merging with approval rules are intermittently failing as discovered in #438159 (closed) & #438158 (closed), this failure doesn't occur all the time and doesn't seem to occur on master(FF related). It seems to be happening quite frequently in staging-canary.

The failure is due to two factors:

1. The test is looking for MWPS rather than auto-merge
2. There is a new bug #438334 (closed)

Fixing either will make the test pass again. This MR addresses point 1.

MWPS has been renamed to be Set to auto-merge. The tests previously waited for MWPS to change to Merge, but since it has been renamed the tests are incorrectly clicking Set to auto-merge. This should be fine, but there is a bug where Set to auto-merge is not working correctly: #438334 (closed), which was found when investigating this issue in #438159 (comment 1728150351)

How to set up and validate locally

QA_COOKIES=gitlab_canary=true QA_LOG_LEVEL=debug GITLAB_QA_USER_AGENT=<1Password> GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD=<1Password> GITLAB_QA_ACCESS_TOKEN=<1Password> QA_GITLAB_URL=https://staging.gitlab.com GITLAB_QA_PASSWORD_1=<1Password> GITLAB_QA_PASSWORD_2=<1Password> GITLAB_ADMIN_USERNAME=gitlab-qa-bot GITLAB_ADMIN_PASSWORD=<1Password> bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/merge_with_code_owner_in_root_group_spec.rb