APIM-7554 fix: enable user search with case-insensitive email address

gravitee-apim-rest-api/gravitee-apim-rest-api-service/src/main/java/io/gravitee/rest/api/service/impl/search/lucene/transformer/UserDocumentTransformer.java

@@ -123,7 +123,9 @@ public Document transform(UserEntity user) {
                 logger.warn("Email of the user {} is not valid", user.getId());
             } else {
                 // For security reasons, we remove the domain part of the email
-                doc.add(new StringField(FIELD_EMAIL, user.getEmail().substring(0, user.getEmail().indexOf('@')), Field.Store.NO));
+                doc.add(
+                    new StringField(FIELD_EMAIL, user.getEmail().substring(0, user.getEmail().indexOf('@')).toLowerCase(), Field.Store.NO)
+                );
             }
         }