Feature Request: Offer an opportunity to rename the site address before a site transfers to Atomic

### What

Default site addresses cannot be changed once they move to the `wpcomstaging.com` subdomain. Barring that changing (related request: https://github.com/Automattic/wp-calypso/issues/87140 ), providing customers an opportunity to consider their site address and update its name _before_ we begin the transfer could help address that limitation.

### Why

Site addresses are routinely created using a customer's username+NUMBER. Some of our onboarding flows can encounter errors and lead to multiple sites getting created. For customers with multiple sites under their account, this can get increasingly confusing, esp when they're not using custom domains yet for whatever reason.

### How

Using the A4A site creation modal as inspiration – could we offer an opportunity to rename the site during the upgrade or "feature activation" process?

For example, we could update this notification:


To provide a field to update that URL, like what we offer for A4A:


Start by locating the A4A site creation modal and the notification used during the upgrade or feature-activation transfer flow. Compare how the existing URL field validates and saves a site address, then define the point before transfer where renaming should occur and verify that the selected address is retained through the transfer.

Written by the indexing model from the issue text.