Replace matching blog address UI

In [Improve "Change Username" Account Settings #94640](https://github.com/Automattic/wp-calypso/pull/94640), we are introducing several tweaks to the Change Username feature to simplify the interfaces.

There's one change we've decided to address in a separate PR related to the "Create a matching site" feature. Whenever a user changes their username, we give them the option to create a blog that matches their new username. The interface for this currently consists of a couple of radio buttons, which are not selected by default:


The user can proceed without making any selection, resulting in only a username change with no site created. This UI is somewhat complex for its purpose, so I'd like to propose using a toggle instead:


Figma file: https://www.figma.com/design/wCm5Tzb7JBaG4WNowWbjES/Logical-Flows?node-id=5250-13523

Start with the Change Username account settings flow described in the linked PR, then compare the current matching-site radio-button interface with the supplied Figma node. Replace that choice UI with the proposed toggle while preserving the username-change and optional site-creation outcomes. Done means the new interface matches the design and both paths remain available.

Written by the indexing model from the issue text.