[2.x] Add upgrade guide

[emmadesilva]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b58f96a) to head (61c2926).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #2319   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      2000      2000           
===========================================
  Files            197       197           
  Lines           5178      5178           
===========================================
  Hits            5178      5178           

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