Implement support for a minimum log level in $wgDebugLogGroups

Implement support for a minimum log level in $wgDebugLogGroups

For some log groups, we only want to log them if they meet a certain
level of severity. An example of this is the current 'memcached-serious'
log group, which can be merged with the normal 'memcached' group
in the future, and report at a severity of ERROR.

This adds a 'level' parameter to the $wgDebugLogGroups, for example:
    $wgDebugLogGroups['memcached'] = array(
        'destination' => '/var/log/mw/memcached.log',
        'level' => \Psr\Log\LogLevel::ERROR,
    );

Bug: T85073
Change-Id: Ic53bc4c8e318ed188fe6f4e838e6789b3c3fd574