[keyserver] Update type for platform in ParsedDelivery

Summary: Should have been done in D8699, after we started putting both ios and macos in the platform field but flow didn't complain so I haven't noticed it.

Test Plan: N/A

Reviewers: marcin, tomek, inka

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8705