chore: Deprecate Scope.user

[sentrivana]

The docstring for Scope.user says it's deprecated in favor of Scope.set_user(), but there is no user-facing warning. Add one so that we can drop the property in the next major.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.57%. Comparing base (ce0727f) to head (57a950f).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4194      +/-   ##
==========================================
+ Coverage   79.55%   79.57%   +0.02%     
==========================================
  Files         141      141              
  Lines       15737    15738       +1     
  Branches     2675     2675              
==========================================
+ Hits        12519    12523       +4     
+ Misses       2371     2369       -2     
+ Partials      847      846       -1     

Files with missing lines	Coverage Δ
sentry_sdk/scope.py	85.59% <100.00%> (+0.01%)	⬆️

... and 2 files with indirect coverage changes