Make function protected that is not used anywhere else

If other code ever wants to use it, we can always change that. But most
probably other code wants to use is_referred_attachment() instead.

program/lib/Roundcube/rcube_message.php

@@ -577,7 +577,11 @@ public function is_attachment($part)
         return false;
     }
 
-    public function get_replacement_references(): array
+    /**
+     * Get a cached list of replacement references, which are collected during
+     * parsing from Content-Id and Content-Location headers of mime-parts.
+     */
+    protected function get_replacement_references(): array
     {
         if ($this->replacement_references === null) {
             $this->replacement_references = [];