[FEATURE] Unified PSR-14 event for modifying the SystemInformationToolbarItem

The SystemInformationToolbarItem now has one unified PSR-14 Event instead
of the previous signals used by SignalSlotDispatcher.

New Event class is called:

- TYPO3\CMS\Backend\Backend\Event\SystemInformationToolbarCollectorEvent

and provides access to the actual SystemInformationToolbarItem allowing
to add messages or new systemInformation blocks by using the API methods
there.

The existing signals are deprecated, and all existing code is migrated
to the PSR-14 event.

Resolves: #89750
Related: #89733
Releases: master
Change-Id: I249c49f8d24b6e2dc73c2db7b4f7c15c9ccfa3fd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62390


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>