make method static and internal

src/Knp/Menu/Twig/MenuExtension.php

@@ -43,7 +43,7 @@ public function getFilters(): array
 
         return [
             new TwigFilter('knp_menu_as_string', $legacy ? [$this, 'pathAsString'] : [MenuRuntimeExtension::class, 'pathAsString']),
-            new TwigFilter('knp_menu_spaceless', [$this, 'spaceless'], ['is_safe' => ['html']]),
+            new TwigFilter('knp_menu_spaceless', [self::class, 'spaceless'], ['is_safe' => ['html']]),
         ];
     }
 
@@ -123,7 +123,10 @@ public function isAncestor(ItemInterface $item, ?int $depth = null): bool
         return $this->runtimeExtension->isAncestor($item, $depth);
     }
 
-    public function spaceless(string $content): string
+    /**
+     * @internal
+     */
+    public static function spaceless(string $content): string
     {
         return trim((string) preg_replace('/>\s+</', '><', $content));
     }