Upstream: special handling of the "Host" header in proxy and gRPC

Upstream: special handling of the "Host" header in proxy and gRPC

HTTP/2 and gRPC requests are now always sent with the ":authority"
pseudo-header, while the "Host" header is no longer passed.
Where a host value is set explicitly with "proxy_set_header Host"
or "grpc_set_header Host", that value is conveyed in ":authority".
As per RFC 9113, Section 8.3.1, clients and intermediaries MUST
generate an ":authority" pseudo-header to convey authority information.

For HTTP/1.1, HTTP/2 and gRPC, if the explicit value is empty, then
the value of $proxy_host is used instead.  As per RFC 9110, Sections
4.2.1 and 4.2.2, the "http" and "https" URI schemes MUST contain a
non-empty host identifier.  The change enforces this rule by
introducing a default.

Closes: https://github.com/nginx/nginx/issues/1426