[TASK] Release version 2.4.0

Push version 2.4.0 to TER.

Add mention of sponsoring to manual.

Resolves: #63347
Releases: 2.4
Change-Id: Id403a4fd95f7013b1e4227d4e1f5d17fdb8c052c
Reviewed-on: http://review.typo3.org/34640
Reviewed-by: Francois Suter <francois@typo3.org>
Tested-by: Francois Suter <francois@typo3.org>