This allows testing Boulder's code to log when old TLS versions are
used.

Update the go in go.mod to 1.24
Update miekg/dns to v1.1.62, matching Pebble.

Unvendor deps: This is a library, so these vendored deps won't get used.

Fixes all lint failures from it

Prepare challtestsrv for the addition of a new DNS-based challenge method, [draft-ietf-acme-dns-persist](https://datatracker.ietf.org/doc/html/draft-ietf-acme-dns-persist-00). Overall functionality is unchanged aside from the API changes documented below; these changes are purely terminology updates and code reorganization.

Rename DNS-01-specific APIs to generic DNS APIs, move TXT records into unified dnsData, and update docs/comments from DNSOne* to DNS*. Note: there are four breaking API changes introduced by this change:

- Config.DNSOneAddrs was renamed to Config.DNSAddrs
- ChallSrv.AddDNSOneChallenge was renamed to AddDNSTXTRecord
- ChallSrv.DeleteDNSOneChallenge was renamed to DeleteDNSTXTRecord
- ChallSrv.GetDNSOneChallenge was renamed to GetDNSTXTRecords

Remove “mock” terminology, merge mockdns.go into dns.go, and update docs/comments accordingly. There are three breaking API changes introduced by this change:

- MockCAAPolicy was renamed to CAAPolicy
- ChallSrv.AddDNSCAARecord now accepts CAAPolicy
- ChallSrv.GetDNSCAARecord now returns CAAPolicy

Restore line that was unnecessarily modified in #31 to satisfy lints to fix an
infinite recursion in challHTTPServer.ListenAndServe() by calling the embedded
server’s method instead of the wrapper itself.

Ensure TXT record values longer than 255 octets are split into multiple RFC 1035
character-string segments when building DNS answers.