move the SupportedVersions slice out of the wire.Header

[marten-seemann]

There's no need to have this slice in the wire.Header. We can reduce the memory size of this struct by treating the supported version as "payload" of a Version Negotiation packet, instead of as part of the header. Not only is this more efficient, it's also more consistent with the QUIC draft.

[codecov]

Codecov Report

Merging #2664 into master will decrease coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2664      +/-   ##
==========================================
- Coverage   86.44%   86.42%   -0.02%     
==========================================
  Files         122      122              
  Lines        9763     9766       +3     
==========================================
+ Hits         8439     8440       +1     
- Misses        989      990       +1     
- Partials      335      336       +1     

Impacted Files	Coverage Δ
internal/wire/version_negotiation.go	86.67% <73.33%> (-13.33%)	⬇️
internal/wire/header.go	92.19% <100.00%> (+0.76%)	⬆️
qlog/qlog.go	95.37% <100.00%> (ø)
session.go	76.67% <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 953f347...993d71f. Read the comment docs.