reduce max DATAGRAM frame size, so that DATAGRAMs fit in IPv6 packets

[marten-seemann]

Fixes #3557.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.50% // No change to project coverage 👍

Coverage data is based on head (d9888df) compared to base (424a663).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3581   +/-   ##
=======================================
  Coverage   85.50%   85.50%           
=======================================
  Files         139      139           
  Lines       10244    10244           
=======================================
  Hits         8759     8759           
  Misses       1103     1103           
  Partials      382      382           

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