dns/knot2: Update from 2.7.6 to 2.8.1

Changelog:

Improvements:

• Possible zone transaction is aborted by zone events to avoid inconsistency
• Added log message if no persistent config DB is available during 'conf-begin'
• New environment setting 'KNOT_VERSION_FORMAT=release' for extended version suppression
• Various improvements in the documentation

Bugfixes:

• Broken NSEC3-wildcard-nonexistence proof after NSEC3 re-salt
• Glue records under delegation are sometimes signed
• RRL doesn't work correctly on big-endian architectures
• NSEC3 not re-salted during AXFR refresh
• Failed to sign new zone contents if added dynamically #641
• NSEC3 opt-out signing doesn't work in some cases
• Broken NSEC3 chain after adding new sub-delegations
• Redundant SOA RRSIG on slave if RRSIG TTL changed on master
• Sometimes confusing log error message for NOTIFY event
• Improper include for LMDB #638

PR: 237371
Submitted by: Leo Vandewoestijne <freebsd@dns.company>
Approved by: Leo Vandewoestijne (maintainer)