net/http: add GODEBUG setting for old ServeMux behavior

net/http: add GODEBUG setting for old ServeMux behavior

Add the GODEBUG setting httpmuxgo121.
When set to "1", ServeMux behaves exactly like it did in Go 1.21.

Implemented by defining a new, unexported type, serveMux121, that
uses the original code.

Updates #61410.

Change-Id: I0a9d0fe2a2286e442d680393e62895ab50683cea
Reviewed-on: https://go-review.googlesource.com/c/go/+/530461
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>