cleanup(otel): bazel flag is obsolete

[dbolduc]

In v1.16.0, OpenTelemetry started compiling with Abseil all the time with Bazel. See https://github.com/open-telemetry/opentelemetry-cpp/pull/2679

This means their --io_opentelemetry_cpp//api:with_abseil flag is now obsolete. Yay. That is good moving forward.

They plan to remove the flag in the future.

The annoying this is that we cannot know which version of OpenTelemetry a downstream target is using. So we will not know if the flag is required or if the flag has been removed. Yuck.

I think our best path forward is to adapt to the change, and try to alert customers to configuration errors through documentation.

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This change is