[Swift6]: Guarantee that unsubscribe to be executed on MainActor

[ra1028]

Checklist

• Reviewed the README and documentation.
• Checked existing issues & PRs to ensure not duplicated.

Description

Currently the unsubscribe call here is not guaranteed to be executed on MainActor.
Using isolated-synchronous-deinit would make it possible to guarantee to be executed on MainActor at compile time without any workaround.

Example Use Case

N/A

Alternative Solution

No response

Proposed Solution

No response

Motivation & Context

No response

[rasberik]

Implemented in 6.1