[BUGFIX] Reduce length of identifier key in sys_file_processedfile

To support a smooth upgrade from a 6.0 or 6.1 system, we have to reduce
the length if the identifier field for the key "identifier".

Resolves: #57002
Releases: 6.2
Change-Id: Ic5928ceae1f288cfb3005ad271e493f356679c82
Reviewed-on: https://review.typo3.org/28622
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind