XWIKI-22820: Solr field cache uses a lot of RAM (#3845)

* Configure the "string" field type with docValues="true" as in the
  default configuration. In tests, this eliminates all fields in the
  field cache apart from the caches for title_sort and
  author_display_sort, which seem small in comparison.

xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-solr/xwiki-platform-search-solr-server/xwiki-platform-search-solr-server-core-search/src/main/resources/conf/managed-schema.xml

@@ -367,7 +367,7 @@
     -->
 
     <!-- The StrField type is not analyzed, but indexed/stored verbatim. -->
-    <fieldType name="string" class="solr.StrField" sortMissingLast="true" />
+    <fieldType name="string" class="solr.StrField" sortMissingLast="true" docValues="true" />
     <fieldType name="strings" class="solr.StrField" sortMissingLast="true" multiValued="true" docValues="true" />
 
     <!-- boolean type: "true" or "false" -->