[TASK] Replace `TYPO3/CMS/Backend/SplitButtons`

[TASK] Replace `TYPO3/CMS/Backend/SplitButtons`

Since FormEngine doesn't use split buttons anymore with TYPO3 v9, the
JavaScript module `TYPO3/CMS/Backend/SplitButtons` has been replaced with
`TYPO3/CMS/Backend/DocumentSaveActions`.

The old module is still in place, but usage will trigger a deprecation
warning in the browser's console.

Resolves: #88651
Releases: master
Change-Id: If180f080b956804d49ca928afe4d86d07d1939bb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61164
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>