[TASK] Make backdrop of modals in Install Tool static

Resolves: #99513
Releases: main
Change-Id: I883007318045e2e7d0ff5e9b0cc7df3d302631cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77325


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>