Site Migration: Hide the 'Do it for me' modal when the user selects the same option previously

[valterlorran]

Related to:

• https://github.com/Automattic/wp-calypso/pull/91856
• https://github.com/Automattic/dotcom-forge/issues/7723

More context: paYKcK-4V1-p2#comment-3677

Proposed Changes

• Displays the Assistance Offer modal only when the user selects the "I'll do it myself" option, which was introduced on https://github.com/Automattic/wp-calypso/pull/91856.
• This modal is displayed when the user presses the Back button on the upgrade step.

Why are these changes being made?

• We discussed on paYKcK-4V1-p2 that maybe the Assistance modal was not necessary when the user selects the "Do it for me" option on the step introduced on https://github.com/Automattic/wp-calypso/pull/91856, as they consciously expressed that they already want the assisted migration.

Testing Instructions

• Apply this PR to your local environment or use the Calypso Live link below.
• Create a free site through the /start flow.
• On the Goal step, select the import option
• Go through the flow until you reach the "How to migrate" step
• Select the "Do it for me" option.
• Once you land on the upgrade step, click on the Stepper Back button and ensure the Assistance modal is not displayed anymore.
• Now select the "I'll do it myself" option.
• Once you land on the upgrade step, click on the Stepper Back button and ensure the Assistance modal is displayed.

Pre-merge Checklist

• [ ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• [ ] Have you written new tests for your changes?
• [ ] Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• [ ] Have you checked for TypeScript, React or other console errors?
• [ ] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [ ] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [ ] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?