Fix UnknownAttributes length assertion

[JoTurk]

Description

Upgrade stun and fix UnknownAttributes length assertion in tests which is now broken because we corrected it in stun pion/stun#280

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.23%. Comparing base (2bd0de0) to head (946de4e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
+ Coverage   82.16%   82.23%   +0.06%     
==========================================
  Files          46       46              
  Lines        3264     3264              
==========================================
+ Hits         2682     2684       +2     
+ Misses        379      378       -1     
+ Partials      203      202       -1     

Flag	Coverage Δ
go	82.23% <ø> (+0.06%)	⬆️

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