Deprecate, and eventually remove, `sentry-types` `protocol` feature

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 [https://github.com/getsentry/sentry-rust/commit/f2612287c6b99389be3916fc08b6570ca4ba4710](), 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.