Remove legacy notification support

[data-sync-user]

parse_chidmessageid includes old code paths for parsing legacy notification chidmessageid formats. This along with similar code in autoush-common’s Notification::chidmessageid and RangeKey::legacy_version should be removed as we no longer support these types of notifications.

Alongside the removal from Notification I think this struct could further enforce topic vs timestamp messages (without the need to support legacy types) by converting the topic and sortkey_timestamp fields into an enum:

struct Notification {
    ...
    ntype: NotificationType,
}

enum NotificationType {
    Topic(String),
    Timestamp(u64),
}

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