[FEATURE] Show warning about duplicate rootPageIds in sites module

The sites module now displays a warning in case the same
root page ID is used for multiple site configurations, as
only the last one will be used.

Resolves: #93423
Releases: main
Change-Id: I984fde19a96ef73e6282f70d45775264c141c147
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76516


Tested-by: Stefan Bürk <stefan@buerk.tech>
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>