[BUGFIX] Add cache StringFrontend to extension scanner

We missed an entry in the extension scanner for the
deprecated cache related StringFrontend. Since that
class tends to be actively used by extensions, it
is worth adding this to the scanner in v9.

Resolves: #87222
Related: #87219
Related: #81434
Releases: 9.5
Change-Id: Ifc2f8d7ef54bc2dea87f428847244669c8435c5a
Reviewed-on: https://review.typo3.org/59219
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>

typo3/sysext/install/Configuration/ExtensionScanner/Php/ClassNameMatcher.php

@@ -949,4 +949,9 @@
             'Deprecation-86366-MethodsInAbstractUpdate.rst'
         ],
     ],
+    'TYPO3\CMS\Core\Cache\Frontend\StringFrontend' => [
+        'restFiles' => [
+            'Deprecation-81434-StringCacheFrontendDeprecated.rst',
+        ],
+    ],
 ];