docs: add Migration Guide to new major v8.0

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (1233bff) to head (05a22b2). Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1404   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            7         7           
  Lines          905       905           
  Branches       295       295           
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  Hits           905       905           

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