Replace h3 parameter with http_version.

[bwelling]

This allows more flexibility; clients can specify which http version they want, or use the default.

[bwelling]

Note that this doesn't add any new tests, because it's unclear how to test. The optimal testing strategy would use nanonameserver, but it doesn't support DoH. Asking real nameservers on the Internet doesn't work, since there's no good way to know what version was used. It might be possible to do some complicated mocking, but I'm not sure how.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.95%. Comparing base (23a6cd2) to head (ba29394).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1068      +/-   ##
==========================================
+ Coverage   93.94%   93.95%   +0.01%     
==========================================
  Files         143      143              
  Lines       13377    13389      +12     
  Branches     2596     2596              
==========================================
+ Hits        12567    12580      +13     
  Misses        477      477              
+ Partials      333      332       -1     

Flag	Coverage Δ
unittests	93.91% <100.00%> (+0.01%)	⬆️

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