[FileUpload, FileDropzone]: rename validateIndividually and change maxSize to bytes #889

Problem

1. validateIndividually JSDoc is incorrect. use-file-upload.ts:51 says "each file is validated separately instead of rejecting all at once." Validation is per-file regardless, and without the flag the batch isn't rejected, valid files still land. It actually means "keep rejected files in the list instead of discarding them", and it's ignored entirely unless multiple is set.
   Also, consider renaming it to something like keepRejectedFiles.
2. Change maxSize to bytes. maxSize is in megabytes (use-file-upload.ts:40-42). Every comparable library takes bytes:

• react-dropzone (Maximum file size (in bytes)),
• PrimeNG maxFileSize,
• Vuetify,
• Syncfusion
• DevExtreme,
• Kendo

As does TEDI's own community tedi-file-dropzone, which additionally grew a
sizeDisplayStandard: "SI" | "IEC" input because of the MB/MiB ambiguity MB invites.