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

This introduces a new PSR-14 event, enabling extension
authors to modify the link explanation, used for TCA
`link` fields in a flexible way.

This replaces the previously available hook
$GLOBALS['TYPO3_CONF_VARS']['SYS']['formEngine']['linkHandler']
which is therefore now removed.

Resolves: #97187
Releases: main
Change-Id: I8937a9870657085bd520d8b813d8d5b4c93f7bc1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73972


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>