Add Rebase without CI button to restructured MR rebase widget

What does this MR do and why?

This adds a Rebase without pipeline button to the restyled MR rebase widget to allow rebasing without triggering a pipeline to be created after the rebase.

The MR rebase widget in the old style already has the second button (see the screenshot below).

Related to #118825 (closed)

Screenshots or screen recordings

Before	After

Existing button when restructured_mr_widget is off

How to set up and validate locally

1. Enable the restructured MR widgets (if not already enabled)

   Feature.enable(:restructured_mr_widget)

2. Enable the rebase without triggering CI feature flag

   Feature.enable(:rebase_without_ci_ui)

3. Go to a project
4. Enable only fast-forward merges (see the docs on how to do this)
5. Create a MR with a small change
6. Create a commit on the default branch
7. See the Rebase without pipeline button on the MR overview

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.