[BUGFIX] Fix logout handling on TYPO3v12

In TYPO3v12, "logoff_post_processing" hook has been deprecated. We migrate to the "BeforeUserLogoutEvent" which allows us to clean up sensitive data from the session as before.

For admin users, the Install tool session handler is invoked during the logout process. The session save path is modified afterwards. To be able to clean the session from sensitive data, we have to store the original session save path in the backend user session.

Changelog: https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.3/Deprecation-100307-VariousHooksRelatedToAuthenticationUsers.html

Resolves: #32 Related: !28