Werk #12155: cmk_update_agent: Fix proxy configuration

When configuring the ruleset "Automatic agent updates" to use a proxy server for connection, the agent updater will fail executing on the host, showing the error:
KeyError: 'server' (visible in the cmk-update-agent.log)

This is fixed now.

This werk is marked as incompatible, because in order to recover from this error, you need to either reinstall the agent on affected hosts after the Checkmk update, or manually add the "server" and "port" entries in the local agent updater proxy configuration at /etc/cmk-update-agent.state (Unix) or config/cmk-update-agent.state (Windows).
E.g., if the "proxy" entry looks like this: "proxy": {"proxy_protocol": "socks5", "user": "some_user", "password": "some_password"},
change it to:
"proxy": {"proxy_protocol": "socks5", "server": "some_server", "port": 1234, "user": "some_user", "password": "some_password"}

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