Make SentrySDKSettings only visible to Swift

[github-actions]

Note

The pull request "Make SentrySDKSettings only visible to Swift" was created by @noahsmartin but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

I noticed the objc compatibility here was only used from tests, so I cleaned up the tests and removed the @objc from SentrySDKSettings.

#skip-changelog

[linear]

COCOA-1036