docs: Mark repo as unmaintained since 2u-phoenix disbanded

[kdmccormick]

The @openedx/2u-phoenix team is empty. Are there new members, or should we delete the team and mark its repos as unmaintained?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.94%. Comparing base (4e29750) to head (8b58d6d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   85.94%   85.94%           
=======================================
  Files         185      185           
  Lines        3879     3879           
  Branches      959      962    +3     
=======================================
  Hits         3334     3334           
  Misses        527      527           
  Partials       18       18           

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