[refactor] make Configuration.config private final

The other two protected properties cannot be made private
because they are accessed from SingleInstanceConfiguration

exist-core/src/main/java/org/exist/util/Configuration.java

@@ -136,9 +136,9 @@ public class Configuration implements ErrorHandler {
     private static final String XQUERY_CONFIGURATION_ELEMENT_NAME = "xquery";
     private static final String XQUERY_BUILTIN_MODULES_CONFIGURATION_MODULES_ELEMENT_NAME = "builtin-modules";
     private static final String XQUERY_BUILTIN_MODULES_CONFIGURATION_MODULE_ELEMENT_NAME = "module";
+    private final Map<String, Object> config = new HashMap<>(); //Configuration
     protected Optional<Path> configFilePath = Optional.empty();
     protected Optional<Path> existHome = Optional.empty();
-    protected Map<String, Object> config = new HashMap<>(); //Configuration
 
     public Configuration() throws DatabaseConfigurationException {
         this(DatabaseImpl.CONF_XML, Optional.empty());