refactor(warnings): report raw signal only, no severity/presentation

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.
Keeps this driver independent of netork and avoids per-vendor drift in
how the same warning code is presented.

napalm_linux/linux.py

@@ -1052,8 +1052,6 @@ class LinuxDriver(OSDriver):
         if updates:
             warnings.append({
                 "code": "updates_available",
-                "severity": "warning",
-                "action": None,
                 "meta": {
                     "count": len(updates),
                     "packages": [u.get("name", "") for u in updates],
@@ -1065,8 +1063,6 @@ class LinuxDriver(OSDriver):
                 if self._apt_proxy_url not in current:
                     warnings.append({
                         "code": "apt_proxy_missing",
-                        "severity": "warning",
-                        "action": "fix_apt_proxy",
                         "meta": {"expected_url": self._apt_proxy_url},
                     })
             except Exception as exc: