[CLEANUP] Follow up: Rename dividers2tabs rst file

dividers2tabs was removed with #62833. The ReST manual file is titled
"Deprecation", but the content says "Breaking". Rename the file.

Resolves: #62844
Releases: master
Change-Id: Id7c9750fe6b204e5ec1995604f6d6d9099b0f1fb
Reviewed-on: http://review.typo3.org/33974
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>