[BUGFIX] Protect a forgotten AbstractFormEngineAjaxController method

With refactoring in #79159 one method has accidently been set
public while it is obvious it should be protected. Fix that. Patch
is not considered to be breaking since the method is an internal
method of that abstract class.

Resolves: #84201
Related: #79159
Releases: master, 8.7
Change-Id: I0c04ff2d3fe9d68420c80af8831df361eb6f8a13
Reviewed-on: https://review.typo3.org/56101
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>

typo3/sysext/backend/Classes/Controller/AbstractFormEngineAjaxController.php

@@ -36,7 +36,7 @@ abstract class AbstractFormEngineAjaxController
      * @param array $result
      * @return array
      */
-    public function createExecutableStringRepresentationOfRegisteredRequireJsModules(array $result): array
+    protected function createExecutableStringRepresentationOfRegisteredRequireJsModules(array $result): array
     {
         if (empty($result['requireJsModules'])) {
             return [];