Change the Merge Immediately button to rightly communicate the action when merge train is enabled
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## 🧩 Problem statement
Without merge trains, **merge immediately** means: `Skip the running pipeline and just merge right away`. WITH merge trains, **merge immediately** means: `Don't make use of the merge train feature and just do a basic merge`. It's almost like those merge requests should have a blue `Start merge train` button, and an additional "green `Merge` button (this is the `merge immediately` drop down button, in reality).
## ⏳ Background
https://gitlab.com/gitlab-org/gitlab/-/issues/320981#note_578018148
## 📝 Proposal
- Change the text of the button that appears as `merge immediately` when merge trains are enabled - `⚠ Skip merge train to merge immediately` and remove the warning icon from the button.
- The confirmation dialogue that appears today should be retained with change in text. Because if a train is running for the project, merging directly will cause the merge train pipelines to restart

- In the settings UI copy under Merge requests>>Enable merge trains, replace `Merge Immediately` to to:
- [ ] **Only allow merging via merge train**
All merge requests must be added to a merge train to merge. The **Skip merge train and merge immediately** option is no longer available in merge requests.
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