don't regard PMTU probe packets as outstanding

[marten-seemann]

This also means that PMTU probe packets won't be declared lost when the PTO timer expires.

[codecov]

Codecov Report

Merging #3126 (5bdbff9) into master (81d16a9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3126      +/-   ##
==========================================
+ Coverage   85.47%   85.48%   +0.01%     
==========================================
  Files         131      131              
  Lines        9650     9651       +1     
==========================================
+ Hits         8248     8250       +2     
+ Misses       1034     1033       -1     
  Partials      368      368              

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_history.go	95.83% <100.00%> (+0.09%)	⬆️
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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