BUG/MEDIUM: qpack: allow 6xx..9xx status codes

BUG/MEDIUM: qpack: allow 6xx..9xx status codes

HTTP status codes outside of 100..599 are considered invalid in HTTP
RFC9110. However, it is explicitely stated that range 600..999 is often
used for internal communication so in practice haproxy must be lenient
with it.

Before this patch, QPACK encoder rejected these values. This resulted in
a connection error. Fix this by extending the range of allowed values
from 100 to 999.

This is linked to github issue #2422. Once again, thanks to @yokim-git
for his help here.

This must be backported up to 2.6.