docs: add migration guide from v6 to v7

[matthewmayer]

I saw there was a migration guide from v5 to v6

https://fakerjs.dev/guide/migration-guide-v5.html

But no such guide for the breaking changes from v6 to v7 described at https://github.com/faker-js/faker/releases/tag/v7.0.0

I thought it would be useful to add this as a reference, as I recently upgraded a project and had to do some digging to find details of the changes.

[codecov[bot]]

Codecov Report

Merging #1246 (a22e844) into main (e5951c0) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1246   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files        2156     2156           
  Lines      237025   237025           
  Branches     1006     1008    +2     
=======================================
+ Hits       236126   236136   +10     
+ Misses        878      868   -10     
  Partials       21       21           

Impacted Files	Coverage Δ
src/modules/internet/user-agent.ts	84.39% <0.00%> (+2.64%)	:arrow_up:

[ST-DDT]

Thank you a lot, this migration guide is really overdue.

[ST-DDT]

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