[TASK] Add getter for TemplateService->rootId

[TASK] Add getter for TemplateService->rootId

The rootID holds the page ID where the root sys_template
record was found.

As this is crucial for some hooks this property
should be publically available but is marked as
deprecated during v9 development, a getter is added
to provide a public API.

Resolves: #83619
Releases: master
Change-Id: Ief651f2e2d758350f0620b3104dd47e0c06fa2f4
Reviewed-on: https://review.typo3.org/55415
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>