chore: upgrade dtls2 to version 0.16.0

[JKRhb]

I just released a new version of the dtls2 package with some additional bug fixes. This PR bumps the version number.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.04 🎉

Comparison is base (c6f2bb1) 29.91% compared to head (5962824) 29.96%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage   29.91%   29.96%   +0.04%     
==========================================
  Files          62       62              
  Lines        2811     2807       -4     
==========================================
  Hits          841      841              
+ Misses       1970     1966       -4     

Impacted Files	Coverage Δ
lib/src/coap_client.dart	0.00% <0.00%> (ø)
lib/src/coap_constants.dart	0.00% <0.00%> (ø)
lib/src/coap_observe_client_relation.dart	0.00% <0.00%> (ø)
lib/src/coap_request.dart	37.14% <0.00%> (ø)
lib/src/stack/layers/observe.dart	0.00% <0.00%> (ø)

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[shamblett]

OK thanks.