rpm_digest default md5 needs updated to sha256.

[rpm_digest default](https://github.com/bernd/fpm-cookery/blob/ae3d72d7428b5b203a95c5f7580f70838cebe046/lib/fpm/cookery/package/package.rb#L31) should be updated to at least sha256. This is the default across any modern OS's and also the fix when RPM packages are built and used on a FIPS OS. md5 is no longer recommended.