MagicDNS no longer requires nameservers

[dotlambda]

According to https://tailscale.com/kb/1081/magicdns#accessing-devices-over-magicdns,

MagicDNS does not require a DNS nameserver if running Tailscale v1.20 or later.

• [x] read the CONTRIBUTING guidelines
• [ ] raised a GitHub issue or discussed it on the projects chat beforehand
• [ ] added unit tests
• [ ] added integration tests
• [x] updated documentation if needed
• [ ] updated CHANGELOG.md