Fix Javadoc warnings due to Java 21 update (+ some other warnings)

The default implicit constructor must also have a doc comment, so I have to make it explicit and either properly restrict the visibility of the constructor, or actually document it.

pandalib-permissions/src/main/java/fr/pandacube/lib/permissions/PermissionExpressionParser.java

@@ -119,5 +119,7 @@ public class PermissionExpressionParser {
 			return super.evaluate(constant, evaluationContext);
 		}
 	}
+
+	private PermissionExpressionParser() {}
 	
 }

pandalib-permissions/src/main/java/fr/pandacube/lib/permissions/Permissions.java

@@ -199,5 +199,7 @@ public class Permissions {
 		checkInitialized();
 		return backendReader.getFullPermissionsList();
 	}
+
+	private Permissions() {}
 	
 }