[BUGFIX] Use non-static method getCache in Avatar class

The getCache method is not static but called as static twice.
Use $this->getCache() instead.

Resolves: #85879
Releases: master,8.7
Change-Id: Icfb28989c201894d0bfd2867642d7703426fabc2
Reviewed-on: https://review.typo3.org/57935
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/backend/Classes/Backend/Avatar/Avatar.php

@@ -59,7 +59,7 @@ public function render(array $backendUser = null, int $size = 32, bool $showIcon
             (string)$showIcon
         );
 
-        $avatar = static::getCache()->get($cacheId);
+        $avatar = $this->getCache()->get($cacheId);
 
         if (!$avatar) {
             $this->validateSortAndInitiateAvatarProviders();
@@ -71,7 +71,7 @@ public function render(array $backendUser = null, int $size = 32, bool $showIcon
                 'backendUser' => $backendUser
             ]);
             $avatar = $view->render();
-            static::getCache()->set($cacheId, $avatar);
+            $this->getCache()->set($cacheId, $avatar);
         }
 
         return $avatar;