Add SolrConfig ClassicIndexSchemaFactory to prevent Solr startup warning.

This overrides Solr default to use managed schema. As we don't use
programatic schema changes this directs Solr to use schema.xml, eliminating
the warning.

defaults/solr/solrconfig.xml

@@ -120,6 +120,18 @@
   <directoryFactory name="DirectoryFactory" 
                     class="${solr.directoryFactory:solr.NRTCachingDirectoryFactory}"/>
 
+  <!-- An alternative to using a managed schema is to explicitly configure a ClassicIndexSchemaFactory.  
+       ClassicIndexSchemaFactory requires the use of a schema.xml configuration file, 
+       and disallows any programatic changes to the Schema at run time.  
+       The schema.xml file must be edited manually and is only loaded only when the collection is loaded.
+       When a <schemaFactory/> is not explicitly declared in a solrconfig.xml file, 
+       Solr implicitly uses a ManagedIndexSchemaFactory, which is by default "mutable" and 
+       keeps schema information in a managed-schema file.
+       
+       YaCy specific setting, overrideing Solr default (introduced YaCy v1.921)
+  -->
+  <schemaFactory class="ClassicIndexSchemaFactory"/>
+    
   <!-- The CodecFactory for defining the format of the inverted index.
        The default implementation is SchemaCodecFactory, which is the official Lucene
        index format, but hooks into the schema to provide per-field customization of