[!!!][FEATURE] Add PSR-14 ModifyButtonBarEvent

The new PSR-14 event ModifyButtonBarEvent is added
as a direct replacement for the now removed hook
$TYPO3_CONF_VARS[SC_OPTIONS][Backend\Template\Components\ButtonBar][getButtonsHook]

Resolves: #96806
Releases: main
Change-Id: Ib743cc532a78a3e38c96e7a89ba2e3856d8cbd0d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73380


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>