chore(issues): Remove legacy browser flag from BE

[leeandher]

Follow up to #67715

Removes organizations:legacy-browser-update and logic from the backend.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.72%. Comparing base (889f77e) to head (9cbb6c5).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #67717       +/-   ##
===========================================
+ Coverage   58.23%   79.72%   +21.49%     
===========================================
  Files        6416     6424        +8     
  Lines      284385   284634      +249     
  Branches    49002    49026       +24     
===========================================
+ Hits       165608   226933    +61325     
+ Misses     118373    57302    -61071     
+ Partials      404      399        -5     

Files	Coverage Δ
src/sentry/api/helpers/default_inbound_filters.py	100.00% <100.00%> (+8.33%)	⬆️
src/sentry/conf/server.py	89.22% <ø> (ø)
src/sentry/features/temporary.py	100.00% <ø> (ø)

... and 1950 files with indirect coverage changes