The golang.org/x/net/http2/h2c package was deprecated in v0.54.0 in
favour of setting the http.Server Protocols field to enable unencrypted
HTTP/2.
This replaces the h2c.NewHandler wrapping added in e863f751f with
http.Server.Protocols, which is supported by the standard library
since Go 1.24.
Note that the stdlib only supports HTTP/2 prior-knowledge on cleartext
connections, not HTTP/1.1 Upgrade: h2c negotiation. In practice clients
use prior-knowledge or require TLS, so this should not affect users.
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