Remove automatic propagator setup from OTLPIntegration

The OTLP integration currently sets up a Sentry propagator by default. This injects `sentry-trace` and `baggage` headers on outgoing requests alongside, or instead of, the standard W3C `traceparent` header.

Per the [OTLP integration spec](), the integration must not set up an automatic propagator. Cross-service trace propagation should be handled by `propagateTraceparent` or by user-configured OpenTelemetry propagators.

Docs:
Update docs to reflect the breaking change and manual propagator configuration path. The planned removal notice is already present in the OTLP docs.

Migration path:
Users who still need Sentry-specific headers in mixed Sentry-native and OpenTelemetry environments can configure propagators manually via the OpenTelemetry propagation API.

Start by locating OTLPIntegration, its tests, and the OTLP documentation, then inspect where the default Sentry propagator is configured. Verify that automatic propagator setup is removed, that propagation remains available through propagateTraceparent or user-configured OpenTelemetry propagators, and that the breaking-change migration path is documented.

Written by the indexing model from the issue text.