rename the SERVER_BUSY error to CONNECTION_REFUSED

[marten-seemann]

See quicwg/base-drafts#3694.

[codecov]

Codecov Report

Merging #2596 into master will not change coverage.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master    #2596   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files         122      122           
  Lines        9755     9755           
=======================================
  Hits         8416     8416           
  Misses        997      997           
  Partials      342      342           

Impacted Files	Coverage Δ
server.go	84.02% <66.67%> (ø)
internal/qerr/error_codes.go	100.00% <100.00%> (ø)
qlog/types.go	84.85% <100.00%> (ø)

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