Add migration guide for future v8

[matthewmayer]

Clear and concise description of the problem

Replace https://next.fakerjs.dev/guide/upgrading.html with migration guide for v7->v8.

Appreciate v8 is not finished yet but I think its probably easier to write the migration guide as we go along, rather in one go at the end, to get an overview of how many breaking changes there are, and so how tricky upgrading will be for end-users.

Note this generally only includes removals, not just deprecations, plus the big faker.person and faker.location changes.

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