Replace Device.Timezone property

There's currently a TODO item in `Sentry.Protocol.Device` [added by Alexey here]().

[https://github.com/getsentry/sentry-dotnet/blob/e75d537c14ded617c99eb33f19cf4ce69ca4de36/src/Sentry/Protocol/Device.cs#L18-L26]()

A serialize -> deserialize can theoretically lose some information at the moment. See:
[https://github.com/getsentry/sentry-dotnet/blob/e6c1d6491a7b2af9f1d0e1d0944b33778cb3b8fe/test/Sentry.Tests/Protocol/Context/DeviceTests.cs#L199-L243]()

This is likely only a problem with custom time zones. We haven't yet bumped into anyone using these. So we agreed not to address this until/unless it becomes a problem for one of our customers.