[DependencyInjection] Deprecate ExtensionInterface::getXsdValidationBasePath() and getNamespace()

Author: nicolas-grekas

Tests/KernelTest.php

@@ -417,11 +417,17 @@ public function load(array $configs, ContainerBuilder $container): void
                 $container->setParameter('test.extension-registered', true);
             }
 
+            /**
+             * @deprecated since Symfony 7.4, to be removed in Symfony 8.0 together with XML support.
+             */
             public function getNamespace(): string
             {
                 return '';
             }
 
+            /**
+             * @deprecated since Symfony 7.4, to be removed in Symfony 8.0 together with XML support.
+             */
             public function getXsdValidationBasePath(): string|false
             {
                 return false;