T6169: DNS forwarding should allow underscore for srv record

This srv recors looks valid:

set service dns forwarding authoritative-domain _tcp.db.mongors1.example.com records srv _mongodb entry 0 hostname 'mongors1.example.com'

But FQDN validator cannot validate it correctly, use regex to fix

(cherry picked from commit 3c37b6a44dca552da950b5288a30c7e074d58704)