[TASK] Declare EXT:backend Event classes and properties readonly

Declare EXT:backend Event classes, only
having readonly properties, as readonly.

Additionally, adds readonly to properties,
which previously missed the declaration.

Resolves: #102789
Releases: main
Change-Id: Ic88d7243bce82844b50462dacaf8381333301fed
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82409


Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>