fix(files): remove invalid FileObject requirements

[ryanduguid]

Summary

• remove four undefined, lower-case required fields from the Files FileObject schema
• allow the documented partial PUT /Files/{FileId} request body (for example, { "FolderId": "..." })

FileObject is shared by file responses and the partial-update request body. Replacing the fields with Name/Id would still make valid partial updates invalid, so this removes the stale required list instead.

Verification

• parsed the OpenAPI YAML with PyYAML
• validated the documented FolderId-only PUT example against FileObject with jsonschema
• npx -y @redocly/cli lint xero_files.yaml --format stylish: removes the four FileObject no-required-schema-properties-undefined warnings (the file retains unrelated baseline lint findings)