[TASK] Allow upper compatibility limit to 7.99.999

Due to the version naming change in TYPO3 7, extensions authors
should be allowed to upload their extensions as compatible with
".99.999" as soon as the corresponding branch is marked as active.

Change-Id: Id2e2c0af535a09461f4d88131721d6517c97a599
Resolves: #63444
Reviewed-on: http://review.typo3.org/34807
Reviewed-by: Thomas Löffler <loeffler@spooner-web.de>
Tested-by: Thomas Löffler <loeffler@spooner-web.de>