[TASK] Mark several methods within TemplateService as internal

The following methods will trigger a deprecation warning if
they are called from the outside:

- TemplateService->prependStaticExtra()
- TemplateService->versionOL()
- TemplateService->processIncludes()
- TemplateService->mergeConstantsFromPageTSconfig()
- TemplateService->flattenSetup()
- TemplateService->substituteConstants()

Several other methods have been marked as internal.

Resolves: #86439
Releases: master
Change-Id: Ifabbda8048449e6919d054da4111f01065a5eeab
Reviewed-on: https://review.typo3.org/58456


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>