Change default DHCP domain to internal

[Self-Hosting-Group]

as current domain is blocked by DNSSEC validating clients

This PR changes the domain, updates the help and also adds it to the new setup wizard. And don't just suggest the name as in #5898, update the default as previously suggested in #7193.

systemd-resolved has internal on its DNSSEC negative trust anchor list since 2016, where home.arpa was added later:
systemd/systemd@30c7780#diff-23206c93f79c419c8a911a58d533e3ca9a6103ff215a5fcc887aaecb59021276R155

.internal history

Internal was first mentioned in RFC 6762 (2013) within multicast DNS:
https://datatracker.ietf.org/doc/html/rfc6762#appendix-G

IANA assessment (2024-01):
https://itp.cdn.icann.org/en/files/root-system/identification-tld-private-use-24-01-2024-en.pdf

ICANN board resolution (2024-07):
https://www.icann.org/en/board-activities-and-meetings/materials/approved-resolutions-special-meeting-of-the-icann-board-29-07-2024-en#section2.a

IETF RFC draft (2025-03):
https://datatracker.ietf.org/doc/html/draft-davies-internal-tld
According to the last IETF 122 meeting there is Almost no work to do:
https://datatracker.ietf.org/doc/minutes-122-dnsop/

.home.arpa (former aproch/name)
https://datatracker.ietf.org/doc/html/rfc8375

[Self-Hosting-Group]

1. Should we slightly improve the current help text to mention DNSSEC validating clients?
2. Should this change also be applied to the 24.7 branch/setup wizard?

[fichtner]

1. defaults are better than lengthy help texts
2. definitely not the stable branch, because we avoid reissuing images on stable branches so no need to taint the code there

[Self-Hosting-Group]

I have updated the PR without using markup and an updated suggestion for the help text.

1. defaults are better than lengthy help texts

1. I have shortened the list of suggested domains, removing the normal (net/com) domains as they can cause problems with DNSSEC validating clients, and only mentioning internal and name.internal. Or home.arpa as well?
2. I have added a short note about DNSSEC issues, is it too long or would it be better to leave it out completely?

Or maybe even shorter:
Do not use 'local' as internal domain name, as reserved for mDNS and will interfere with mDNS (avahi/bonjour). Use special-use domain internal, also to avoid DNSSEC issues. E.g. internal, name.internal.

[fichtner]

Merged, thanks!