[TASK] Rename composer.json to lowercase

Unix is a case sensitive filesystem, with the composer command
relying on an existing lowercase composer.json in order to work
correctly. This change will cause no trouble on OSX, but enables
the command on Unix.

Change-Id: I21ca0fe140643164ac11874f195f30be47e1e486
Resolves: #56680
Releases: 6.2
Reviewed-on: https://review.typo3.org/28189
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes