[TASK] Make Extbase use cache group "pages"

As there is the possibility to use grouped caching in
order to be more flexible, Extbase should not use
hard-coded cache names for clearing the frontend
cache (cache_pagesection and cache_pages) but use
the group, so other caches could be triggered as well.

Releases: 6.2
Resolves: #55044
Change-Id: I8077f889fce7561170bffc7f97c1884c698d673c
Reviewed-on: https://review.typo3.org/26866
Reviewed-by: Felix Oertel
Tested-by: Felix Oertel