14857: prevent cache drop for frontend caches on sitemap generation

#14857

Introduces a (dummy) cache tag for frontend in sitemap and robots generation. This prevents the
default and page_cache to be dropped completely. The cache tag for full_page cache is not
modified.

Author: david-fuehr

app/code/Magento/Robots/Model/Config/Value.php

@@ -37,7 +37,7 @@ class Value extends ConfigValue implements IdentityInterface
      * @var string
      * @since 100.2.0
      */
-    protected $_cacheTag = true;
+    protected $_cacheTag = [self::CACHE_TAG];
 
     /**
      * @var StoreManagerInterface

app/code/Magento/Sitemap/Model/Sitemap.php

@@ -159,7 +159,7 @@ class Sitemap extends \Magento\Framework\Model\AbstractModel implements \Magento
      * @var string
      * @since 100.2.0
      */
-    protected $_cacheTag = true;
+    protected $_cacheTag = [Value::CACHE_TAG];
 
     /**
      * Item resolver