[TASK] Set TYPO3 version to 4.7.21-dev

Change-Id: I129536ec1c665f0446f5d910e00fe941d2d63236
Reviewed-on: http://review.typo3.org/33464
Reviewed-by: TYPO3 Release Team <typo3v4@typo3.org>
Tested-by: TYPO3 Release Team <typo3v4@typo3.org>