[TASK] Declare Event classes and properties readonly

Declare Event classes, only having readonly
properties, as readonly.

Resolves: #102795
Releases: main
Change-Id: I1d2de9a0f95dba7eece5ccafb858a697c1eb4420
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82412


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>