refactor(warnings): report raw signal only, align meta key naming

get_device_warnings() now returns only {code, meta} — severity, title,
message, and action are resolved centrally by netork's
WARNING_CATALOG (netork/core/device_warnings.py), not by the driver.
Also renames meta.version -> meta.latest_version for
firmware_update_available so it matches the netgear driver's shape and
the shared catalog message template.

napalm_fritzbox/fritzbox.py

@@ -411,9 +411,7 @@ class FritzBoxDriver(ResidentialGatewayDriver):
                 if ui_info.get("NewX_AVM-DE_UpdateState", "") == "Available":
                     warnings.append({
                         "code": "firmware_update_available",
-                        "severity": "info",
-                        "action": None,
-                        "meta": {"version": ui_info.get("NewX_AVM-DE_Version", "")},
+                        "meta": {"latest_version": ui_info.get("NewX_AVM-DE_Version", "")},
                     })
             except _OPTIONAL_SERVICE_ERRORS:
                 pass
@@ -424,12 +422,7 @@ class FritzBoxDriver(ResidentialGatewayDriver):
                 # SNR margin is in dB×10; < 60 means < 6 dB — borderline line quality
                 snr_down = int(_to_float(info.get("NewDownstreamNoiseMargin")))
                 if 0 < snr_down < 60:
-                    warnings.append({
-                        "code": "dsl_snr_low",
-                        "severity": "warning",
-                        "action": None,
-                        "meta": {"snr_db_x10": snr_down},
-                    })
+                    warnings.append({"code": "dsl_snr_low", "meta": {"snr_db_x10": snr_down}})
             except _OPTIONAL_SERVICE_ERRORS:
                 pass