Get rid off dbus_proxy warning about defaults

[pnevyk]

zbus introduced assume_defaults attribute for dbus_proxy macro in version 3.5.0. They also started to print a warning if neither of assume_defaults/default_path/default_service were specified, which for keepawake looked like this:

#[dbus_proxy(...)] macro invocation on 'ScreenSaverProxyBlocking' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.
#[dbus_proxy(...)] macro invocation on 'ScreenSaverProxy' without explicit defaults. Please set 'assume_defaults = true', or configure default path/service directly.

This PR updates the zbus dependency to 3.5.0 and sets assume_defaults = true which is, according to docs, the current default. Hence the warning is gone.