config, types: move randomize_client_port from server config to policy file

Tailscale models the randomize-client-port toggle as a top-level
field on the ACL policy. Headscale now matches that shape: the
server-config randomize_client_port key is removed, the toggle
lives in the policy file as randomizeClientPort, and per-node
opt-in via nodeAttrs is also supported.

Operators upgrading from a config-set randomize_client_port hit
depr.fatalWithHint at startup, which prints the deprecation message
and points at the new policy field rather than silently dropping
the toggle. The default carries over (false) so operators who never
set it are unaffected. config-example.yaml ships a REMOVED stanza
showing the migration.

types/node.go drops the cfg.RandomizeClientPort read from
TailNode -- the cap is now policy-driven through compileNodeAttrs
and the tail_test.go expectations follow.