Appendix C: Raise file size softlimit

[tschmidtb51]

The practice has shown that the current file size soft limit, given through Appendix C.1 does not meet the needs in practice. At least Red Hat is generating files that are bigger than 15 MB.
Therefore, I suggest to raise the file size limit to 50 MB.

[sthagen]

During the 2023-09-27 meeting of the TC the members approved to raise the limit to 50 Megabytes as proposed in CSAF v2.1.