cmd/geth: enable DNS discovery by default (#20660)

* node: expose config in service context

* eth: integrate p2p/dnsdisc

* cmd/geth: add some DNS flags

* eth: remove DNS URLs

* cmd/utils: configure DNS names for testnets

* params: update DNS URLs

* cmd/geth: configure mainnet DNS

* cmd/utils: rename DNS flag and fix flag processing

* cmd/utils: remove debug print

* node: fix test