add support for Go 1.18

[marten-seemann]

... based on the recently release beta.1.

[codecov]

Codecov Report

Merging #3298 (ba4d020) into master (b935a54) will decrease coverage by 0.02%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master    #3298      +/-   ##
==========================================
- Coverage   85.69%   85.66%   -0.02%     
==========================================
  Files         132      133       +1     
  Lines        9774     9786      +12     
==========================================
+ Hits         8375     8383       +8     
- Misses       1024     1028       +4     
  Partials      375      375              

Impacted Files	Coverage Δ
internal/qtls/go117.go	58.33% <ø> (ø)
internal/qtls/go118.go	58.33% <58.33%> (ø)
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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