[BUGFIX] Fix composer branch alias of master branch

Branch alias of master branch still is to 7.x-dev,
but obviously must be 8.x-dev

Resolves: #73104
Releases: master
Change-Id: I694b991600a98ba28cadc83cf718d5620b494768
Reviewed-on: https://review.typo3.org/46462


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Christoph Buchli <cbuchli@snowflake.ch>
Tested-by: Christoph Buchli <cbuchli@snowflake.ch>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>