Update Subscription Sync enums and specs

What does this MR do?

This MR is doing a bit of polishing after !65316 (merged) got merged. Given the nature of the subscription sync (which happens async), strictly speaking, the frontend has no knowledge of successful sync, only that the sync started and it's pending. We want to reflect this in how code treats the states of the application updating methods and properties where applicable.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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