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.