feat(rr6): Tie react-router 6 to it's feature flag

[evanpurkhiser]

This will allow us to slow roll react router 6

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files	Patch %	Lines
static/app/index.tsx	0.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #76470      +/-   ##
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- Coverage   78.16%   78.15%   -0.01%     
==========================================
  Files        6894     6894              
  Lines      306749   306752       +3     
  Branches    52794    52795       +1     
==========================================
  Hits       239757   239757              
- Misses      60604    60607       +3     
  Partials     6388     6388