chore: deprecation message for problem-builder

[xitij2000]

This change adds a message deprecating problem builder.

Announcement on forums: https://discuss.openedx.org/t/deprecation-removal-problem-builder-repository/13664
DEPR Ticket: openedx/public-engineering#268

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.04%. Comparing base (a4cfa28) to head (61ab87d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #420   +/-   ##
=======================================
  Coverage   45.04%   45.04%           
=======================================
  Files          40       40           
  Lines        3303     3303           
  Branches      401      401           
=======================================
  Hits         1488     1488           
  Misses       1768     1768           
  Partials       47       47           

Flag	Coverage Δ
unittests	45.04% <100.00%> (ø)

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