Drop redundant dynamic cluster mode

The dynamic mode produced a byte-identical cluster to authoritative:
RFC2136 update capability is a per-zone property (BindZone.dynamicUpdate
+ updateKeyRef render allow-update { key ... }), not a cluster role. The
external-dns tier is just an authoritative cluster whose zones allow
dynamic updates.

- remove BindMode dynamic from the enum and constants
- switch the external-dns sample to mode authoritative
- update README migration mapping

config/crd/bases/bind.unkin.net_bindclusters.yaml

@@ -1007,7 +1007,6 @@ spec:
                 enum:
                 - authoritative
                 - resolver
-                - dynamic
                 type: string
               nodeSelector:
                 additionalProperties: