update initial salts and test vectors for draft-29

[marten-seemann]

See quicwg/base-drafts#3711.

We'll merge this PR just before updating quic-go for draft-29 (that will make it easier to cut further patch releases for draft-28).

[codecov]

Codecov Report

Merging #2587 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2587   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files         122      122           
  Lines        9755     9755           
=======================================
  Hits         8416     8416           
  Misses        997      997           
  Partials      342      342           

Impacted Files	Coverage Δ
internal/handshake/initial_aead.go	100.00% <ø> (ø)
internal/handshake/retry.go	71.43% <100.00%> (ø)

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