[BUGFIX] Index on sys_file_processedfile is still too large

On some systems even "332" characters is too large. To also cope with
potential utf8mb4 setups (1 char = 4 bytes), and staying under 1000 byte
limit, we reduce the amount of characters in the index to 249.

Resolves: #56786
Related: #54234
Releases: 6.2
Change-Id: I79452224f37ec2b6ca5b3a35044144eed787df67
Reviewed-on: https://review.typo3.org/28429
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny