[TASK] Introduce "TYPO3 CMS" in EXT: about

Changed terminology to "TYPO3 CMS" where applicable in
backend module About.

Change-Id: Id1e9aba5eedaa9da218d3887a71854cf03bf09c1
Resolves: #41823
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/15473
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner