Deprecate, and eventually remove, sentry-types protocol feature #1133

The sentry-types crate has protocol feature, which is enabled by default.

However, the crate does not compile without the protocol feature, and it seems it likely has not compiled without that feature since f261228, released in 0.20.0 almost six years ago as of writing.

I could not find any issues about this compile error in that time, so I think we can safely conclude that there is no need for a sentry-types crate without the protocol feature. Therefore, let's stop gating things with the feature flag, so that it becomes unnecessary: sentry-types without protocol should become equivalent to today's sentry-types with protocol. We can then stop using the protocol feature, deprecate it (perhaps by emitting a compile warning when enabled, not sure the best way to do this), then we can remove the feature in a major release.