dns/dnsmasq-devel: update to v2.91rc6
...which is the same as v2.91.
ChangeLog: https://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=blob_plain;f=CHANGELOG;hb=v2.91rc6
Not updating to v2.92test1 yet because we see no need to upgrade
the dns/dnsmasq port to v2.91 and so dnsmasq-devel can reach a
stable end in 2025Q1.