MR widget instructions are unclear when merge trains are enabled

What does this MR do?

When a project has merge trains enabled, the initial state of the merge button widget after a merge request has been created contains contradictory information.

To make this clearer, we do the following:

• Change the ! icon for the pipeline loading.
• Move This merge request will start a merge train when pipeline # succeeds from its current position to right under the button.
• Change the text for This merge request will start a merge train when pipeline # succeeds for This action will start a merge train when pipeline # succeeds

To be able to change the section from the bottom to the top, I had to move the component from mr-widget-options to ready-to-merge widget and adjust the tests as well.

Screenshots

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #218084 (closed)