ref: Remove SDK_V9 checks around SentrySerializable

[github-actions]

Note

The pull request "ref: Remove SDK_V9 checks around SentrySerializable" was created by @noahsmartin but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

I was hoping to remove SentrySerializable from the public API in V9, however attempts at doing so hit compiler issues in the cocoapods setup that I haven't been able to resolve yet: #6522

There are other higher priority changes for V9 so I suggest we just remove these preprocessor checks and focus on the other V9 changes, we can always come back to this if there's time later

[linear]

COCOA-863