Change log level in KV store GC

Since NXP-30851, the method KeyValueBlobTransientStore#computeStorageSize display a WARN message to indicate the storage size for a computation

        log.warn("Storage size for {}: {} bytes", name, size);

=> it makes this message displayed X times every 15 minutes (X depends on the number of computations configured on the Nuxeo node)

The request is to change the log level to INFO to prevent from printing the message to the logs by default.