feat!: report the package proxy as a fact; no warnings of its own (0.9.0)

get_device_warnings decided two things for the application: that any
pending update is a warning (it re-read the updates for that), and
whether apt's proxy is the one the application's setting wants, which
it was handed as optional_args["apt_proxy_url"].

The driver reports facts instead (napalm-device-types 4.14):
get_package_proxy() says which proxy apt fetches through, None for none
or on a non-apt host; the updates are get_available_updates() as
before. get_device_warnings and the apt_proxy_url argument go.

BREAKING CHANGE: get_device_warnings and optional_args["apt_proxy_url"]
are removed. Requires napalm-device-types 4.14.

Closes #28

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **Breaking:** `get_device_warnings` is gone, and with it the `apt_proxy_url` argument.
+  The driver reports facts and the application names warnings (napalm-device-types 4.14):
+  `get_package_proxy()` says which proxy apt fetches through; pending updates were always
+  in `get_available_updates()`. Requires napalm-device-types 4.14.0; version 0.9.0.
+
 ### Changed
 
 - **Breaking:** `run_device_action` is gone, with `fix_snmp`, `fix_apt_proxy`,