BUG/MINOR: mux-h2: strip the userinfo when deriving :authority for a server

BUG/MINOR: mux-h2: strip the userinfo when deriving :authority for a server

The H2 mux properly drops userinfo from authority on input but doesn't
drop it on output if present on input (e.g. coming from H1), which will
cause a bad request when reaching a compliant H2 server such as itself.
Let's make sure it is properly dropped there as well, as required by
RFC9113. This should be backported to all stable versions.

Reported-by: Claude (ANT-2026-R03JNY63)