Remove `enableTracing` (and update docs accordingly)

[HazAT]

Currently, we offer multiple options to configure sampling for tracing:

• tracesSampleRate
• tracesSampler
• enableTracing

But when to use which, is sometimes not obvious to our users. What makes this worse is that implications of some of them differ across SDKs, for example when it comes to Trace Propagation. In Python for example, propagation of trace headers works without any tracing config options in place being set. In Go, you need to at least set enableTracing to true in order to achieve the same result.

Goal is:

• remove enableTracing in all SDKs
• make tracesSampleRate the default, also in config snippets
• explain how to get more out of sampling by using the tracesSampler function
• mark enableTracing in all SDK docs as deprecated (config option page)

Deprecation

## SDKs
- [ ] https://github.com/getsentry/sentry-javascript/issues/12883
- [ ] https://github.com/getsentry/sentry-dart/issues/2181
- [ ] https://github.com/getsentry/sentry-cocoa/issues/4177
- [x] PHP SDK https://github.com/getsentry/sentry-php/pull/1743
- [x] Laravel SDK https://github.com/getsentry/sentry-php/pull/1743
- [x] Symfony SDK https://github.com/getsentry/sentry-php/pull/1743
- [x] React Native SDK (via JS https://github.com/getsentry/sentry-javascript/issues/12883)
- [ ] https://github.com/getsentry/sentry-java/issues/3566
- [x] ~Kotlin Multiplatform SDK~
- [x] ~~Native SDK~~
- [x] Android SDK (via Java SDK)
- [x] .NET SDK https://github.com/getsentry/sentry-dotnet/pull/3569
- [x] Unity SDK - Handled via .NET SDK. See above.
- [x] PowerShell SDK - Handled via the .NET SDK. See above.
- [x] Unreal SDK - Handled via Cocoa, Android and Native SDK. See above.
- [x] Python SDK https://github.com/getsentry/sentry-python/issues/3918
- [x] Electron SDK - will happen via JS v9
- [x] Ruby SDK https://github.com/getsentry/sentry-ruby/pull/2535
- [x] Rust SDK
- [x] ~~Capacitor SDK (via JS)~~
- [x] ~~Cordova SDK not on JS v8 yet and adoption very low~~
- [x] Sentry CLI
- [x] Mobile docs: https://github.com/getsentry/sentry-docs/pull/12561

[cleptric]

I don't see an issue with the naming here, what makes tracesSampleRate not apply to a span vs a transaction? We'll always need some sort of rates user's can apply.