Support QUIC v2 (RFC 9369)

[mxinden]

RFC 9369 is functionally identical to v1: only crypto salts, labels and codepoints differ. Goal simply is anti-ossification.

Quinn only accepts QUIC v1 and the legacy draft versions today, so v2's version number 0x6b3343cf is rejected:

• interpret_version maps only V1Draft/V1 and returns UnsupportedVersion for everything else.
• DEFAULT_SUPPORTED_VERSIONS lists only v1 plus the old drafts.
• retry_tag and is_valid_retry only carry the v1 Retry integrity keys and unreachable!() otherwise.

The TLS layer is already there: rustls' quic::Version has a V2 variant and derives the v2 initial keys.

Prior context: v2 was raised as an open question in #1528 but never picked up; the version-negotiation mechanism is tracked in #1235.

Additional context:

• https://mxinden-bot.github.io/slides/04-quic-discussion/#/13