formatter: Change log channel of infobox message
By default, the minimum log level for Logstash is INFO [0]. The minimum
log level for the "mobile" channel, however, is DEBUG [1].
[0] https://github.com/wikimedia/operations-mediawiki-config/blob/8342bcbbd040bab1cd3b140b47a58f7ee08d1a46/wmf-config/InitialiseSettings.php#L4795
[1] https://github.com/wikimedia/operations-mediawiki-config/blob/8342bcbbd040bab1cd3b140b47a58f7ee08d1a46/wmf-config/InitialiseSettings.php#L4868
Bug: T149884
Change-Id: I61d20ea8dcc8cd63e826f56567302248b3c92f61