Upgrading the extension
Upgrading from EXT:picturecredits 2.x to 3.x
The plugin content element (list) and the plugin sub types field (list_) have been marked
as deprecated in TYPO3 v13.4 and were removed in TYPO3 v14.0.
When you upgrade this extension to version 3, you therefore need to execute the upgrade wizard Migrate "Picture credits" plugins to content elements. to automatically migrate existing plugins to the new type.