proxy: don't set app logger level

logging is configured globally in SlapOSApp.configure_logging and:

• if logging to file is enabled, it is logged with maximum level
• logging on the console depends on --quiet / --verbose command line flags

Default log level of this logger was DEBUG, which is just fine: the logger emit all messages and the file/console handler decide to output them or not

Because we configure logger after Flask initialize its own logging, for Flask >= 1.1.2, we remove Flask logger as described in https://flask.palletsprojects.com/en/1.1.x/logging/#removing-the-default-handler

Since this is internal API which doest not exist on old version of Flask, we adjust requirements to require a recent version of Flask.