Application Passwords on Migrations: UI- Refactor credential collection step

[andregardi]

Refactor (based on feature flag) credential collection step to:

• Remove the user/pass form
• Call the new endpoint to check for AP support
• Have a wait state while the endpoint resolves

[github-actions[bot]]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Migration: The issue relates to improving the credential collection process during a site migration, indicating a focus on the migration functionality.
• [Feature] Authentication: This issue involves refactoring the credential collection step, which directly pertains to user authentication processes.
• [Feature] API: The mention of calling a new endpoint indicates the need for API interaction in verifying application password support.
• [Feature] User Management: The credential collection step is related to managing user credentials, which falls under user management.