x/net/http2/h2c: uses obsolete upgrade mechanism

[irbekrm]

x/net/http2/h2c package uses HTTP Upgrade headers based mechanism to determine if a connection should be upgraded to h2c.
This mechanism was deprecated by RFC 9113 https://datatracker.ietf.org/doc/html/rfc9113#name-http-2-version-identificatio

Perhaps it makes sense to leave the functionality as is to not break existing clients (?) but would be good to maybe mention the deprecation in code comments and rewrite the other comments that refer to the obsolete RFC 7540 to refer to sections in RFC 9113

[mauri870]

The "h2c" string was previously used as a token for use in the HTTP Upgrade mechanism's Upgrade header field (Section 7.8 of [HTTP]). This usage was never widely deployed and is deprecated by this document. The same applies to the HTTP2-Settings header field, which was used with the upgrade to "h2c".

This usage was never widely deployed I'm not sure If I fully agree with that, h2c seems to be used all over the place

We should be good boys and clarify about it being obsolete with a link to the RFC9113, which is the authoritative source.

[cagedmantis]

cc @neild @tombergan

[seankhliao]

The new h2c in the standard library no longer supports Upgrade: h2c, see #67816