[BUGFIX] Add missing css class to create new IRRE relations

This patch adds the missing css class to get the "add new relation" button for
IRRE selector fields working again.

Resolves: #91839
Releases: master, 10.4
Change-Id: I1cb3ff34db608eb826f4e351850bf78f2bbe4e27
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65070
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>

typo3/sysext/backend/Classes/Form/Container/InlineControlContainer.php

@@ -640,7 +640,7 @@ protected function renderPossibleRecordsSelectorTypeSelect(array $config, array
             }
             $item .= '
             <span class="input-group-btn">
-                <a href="#" class="btn btn-default" title="' . $createNewRelationText . '">
+                <a href="#" class="btn btn-default t3js-create-new-button" title="' . $createNewRelationText . '">
                     ' . $this->iconFactory->getIcon('actions-add', Icon::SIZE_SMALL)->render() . $createNewRelationText . '
                 </a>
             </span>';