[!!!][TASK] Remove deprecated code in FormProtectionFactory

Removed public methods:
* \TYPO3\CMS\Core\FormProtection\FormProtectionFactory->get
* \TYPO3\CMS\Core\FormProtection\FormProtectionFactory->purgeInstances

Resolves: #101013
Related: #100963
Related: #99098
Releases: main
Change-Id: I5b8b9a82382c0326271d25c763bb2ba52cf78441
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79300


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>