http3: send GOAWAY when server is shutting down

[marten-seemann]

Continuation of #4669. Part of #153.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 36 lines in your changes missing coverage. Please review.

Project coverage is 84.60%. Comparing base (e5693d0) to head (674c49f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
http3/server.go	63.64%	33 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4691      +/-   ##
==========================================
- Coverage   84.79%   84.60%   -0.19%     
==========================================
  Files         150      150              
  Lines       14929    15024      +95     
==========================================
+ Hits        12658    12710      +52     
- Misses       1742     1782      +40     
- Partials      529      532       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[marten-seemann]

This should be ready to go now.