Add mention to DD_TRACE_OTEL_ENABLED

[julianocosta89]

What does this PR do? What is the motivation?

DD_TRACE_OTEL_ENABLED=true
is mentioned under https://docs.datadoghq.com/tracing/trace_collection/custom_instrumentation/java/otel/#setup and should also be called out under https://docs.datadoghq.com/tracing/trace_collection/library_config/java.

Merge instructions

• Please merge after reviewing

[mcculls]

We should remove this from the experimental section at the same time, because DD_TRACE_OTEL_ENABLED replaces it:

dd.integration.opentelemetry.experimental.enabled : Environment Variable: DD_INTEGRATION_OPENTELEMETRY_EXPERIMENTAL_ENABLED
Default: false
Enables experimental OpenTelemetry features. When you enable this setting, you can use the OpenTelemetry tracing API backed by the Datadog Trace Agent.
Available since version 1.10.0

[rtrieu]

Minor suggestion but otherwise looks good. Thanks!

[rtrieu]

Looks good!