Prevent log pollution from unwanted Solr warnings.

Many non-blocking "java.nio.file.NoSuchFileException" traces with
warning log level can be logged by Solr, especially when heavily
crawling. This is issue is known from Solr 5.x but still unresolved with
Solr 6.x ( https://issues.apache.org/jira/browse/SOLR-9120 )

Consequently upgraded to "SEVERE" the default log level of the related
internal Solr class.

See also mantis 727 ( http://mantis.tokeek.de/view.php?id=727 )

defaults/yacy.logging

@@ -99,6 +99,7 @@ org.apache.solr.rest.ManagedResource.level = SEVERE
 org.apache.solr.util.plugin.AbstractPluginLoader.level = INFO
 org.apache.solr.schema.IndexSchema.level = INFO
 org.apache.solr.schema.FieldTypePluginLoader.level = INFO
+org.apache.solr.handler.admin.LukeRequestHandler.level = SEVERE
 org.apache.solr.handler.component.HttpShardHandlerFactory.level = SEVERE
 org.apache.solr.handler.component.QueryElevationComponent.level = SEVERE
 org.apache.solr.handler.component.SearchHandler.level = SEVERE