rename qlog event key_retired to key_discarded

[marten-seemann]

See quicwg/qlog#206.

[codecov]

Codecov Report

Merging #3463 (afd6513) into master (f29dd27) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3463   +/-   ##
=======================================
  Coverage   85.66%   85.66%           
=======================================
  Files         137      137           
  Lines       10009    10009           
=======================================
  Hits         8574     8574           
  Misses       1059     1059           
  Partials      376      376           

Impacted Files	Coverage Δ
qlog/event.go	98.11% <100.00%> (ø)
qlog/qlog.go	91.07% <100.00%> (ø)

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