add support for QUIC draft-34

[marten-seemann]

QUIC draft-34 will (if everything goes according to plan at the IETF) become QUIC v1.

draft-34 uses a different salt for Initial packet encryption, and it changes the TLS extension code point for the QUIC transport parameter extension (yay!).

We're not allowed to deploy this version on the public internet yet. I added a note for SupportedVersions, so we don't forget that when cutting the next release.

[codecov]

Codecov Report

Merging #3031 (615540f) into master (0b97ae5) will decrease coverage by 0.02%.
The diff coverage is 83.87%.

@@            Coverage Diff             @@
##           master    #3031      +/-   ##
==========================================
- Coverage   86.05%   86.04%   -0.02%     
==========================================
  Files         135      135              
  Lines        9472     9489      +17     
==========================================
+ Hits         8151     8164      +13     
- Misses        961      964       +3     
- Partials      360      361       +1     

Impacted Files	Coverage Δ
http3/server.go	68.15% <42.86%> (-0.91%)	⬇️
internal/handshake/crypto_setup.go	66.83% <83.33%> (+0.24%)	⬆️
internal/handshake/initial_aead.go	100.00% <100.00%> (ø)
internal/handshake/tls_extension_handler.go	100.00% <100.00%> (ø)
internal/protocol/version.go	100.00% <100.00%> (ø)
server.go	82.25% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b97ae5...615540f. Read the comment docs.

[marten-seemann]

I got this wrong in part. draft-34 is draft-34, not version 1. Maybe (after IESG review), there'll be a version 1, which will be identical to draft-34 though.

I updated the PR accordingly.