Update migration.md with changed behavior of User.expired

[olenitsj]

Closes/fixes #issue

Checklist

• This PR makes changes to the public API
• I have included links for closing relevant issue numbers

[pamapa]

thanks for improving the migration guide

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1a36b87) 77.82% compared to head (fb0c829) 77.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1128   +/-   ##
=======================================
  Coverage   77.82%   77.82%           
=======================================
  Files          45       45           
  Lines        1718     1718           
  Branches      341      341           
=======================================
  Hits         1337     1337           
  Misses        341      341           
  Partials       40       40           

Flag	Coverage Δ
unittests	77.82% <ø> (ø)

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