Onboarding: Fix going back from Checkout to Stepper

[alshakero]

Fixes: #96888

Reverts https://github.com/Automattic/wp-calypso/pull/95244/files#diff-d6bc20bcdcb4a999ce96bb33bf6f986392311a3800680220300a7477733748f7R101-R112.

Proposed Changes

This corrects a mistake in the onboarding flow that replaces the plans route when navigating to create-site route.

Why are these changes being made?

So that going back from checkout to /plans works in the onboarding flow.

Testing Instructions

• Make sure you can't reproduce Stepper: Browser back from Checkout to Plans does not reach Domains #96888.
• Make sure going back to plans then re-selecting a plan yields the correct plan and domain in checkout.
• Make sure going back to domain, clicking Continue, then re-selecting a plan yields the correct plan and domain in checkout.

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126540

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126540&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-126540&env=a8c-for-agencies

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/going-back-from-checkout on your sandbox.

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[escapemanuele]

LGTM! 👍