chore: update readme on legacy flags

[rayzhou-bit]

Description

A number of features now default to being on in edx-platform. In order to use the legacy versions of these features, legacy flags need to be set. This PR updates documentation to reflect these changes.

Other information

Follows changes made in this PR: openedx/openedx-platform#36495

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (d62c4cf) to head (005da2d).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1861      +/-   ##
==========================================
- Coverage   93.77%   93.76%   -0.01%     
==========================================
  Files        1146     1146              
  Lines       23903    23903              
  Branches     5041     5161     +120     
==========================================
- Hits        22414    22413       -1     
+ Misses       1421     1414       -7     
- Partials       68       76       +8     

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[Faraz32123]

LGTM! 👍