feat: report the firmware Netgear offers, not a warning (0.4.0)

The Plus driver looked the newest firmware up in Netgear's download
catalogue and named firmware_update_available itself. It now reports
get_firmware_offer() (napalm-device-types 4.16 FirmwareOfferMixin): what
runs, the newest offered, whether that is newer by Netgear's versioning,
and the download link; netOrk names the warning (netOrk #1111). The Smart
driver's empty get_device_warnings goes too. Requires
napalm-device-types 4.16.0.

napalm_netgear/netgear_smart.py

@@ -1838,10 +1838,6 @@ class NetgearSmartDriver(ConfigLifecycleMixin, SnmpAgentMixin, SwitchDriver):
 
     # ── SNMP / Health ──────────────────────────────────────────────────────────
 
-    def get_device_warnings(self) -> list:
-        """Return device warnings. SNMP detection handled by poll task."""
-        return []
-
     def get_snmp_config(self):
         """Return SNMP config if a community is configured on the switch."""
         try: