Add policy to prevent enum member removal

[lmolkova]

See

https://github.com/open-telemetry/semantic-conventions/actions/runs/12331081275/job/34417268501?pr=1684

[jsuereth]

The policy exists, but only applies to stable enums:
https://github.com/open-telemetry/semantic-conventions/blob/main/policies/compatibility.rego#L219

Do you want it to apply to unstable enums too?

[lmolkova]

The practical reason for keeping deprecated experimental attributes is to be able to generate one semconv artifact that can work with N instrumentation libraries potentially following different versions of semconv.

If we drop an attribute and instrumentation gets unexpected version of semconv artifact resolved in runtime there will be a runtime exception.

Enum members would have the same problem, but maybe with a smaller probability.

So I do think we should keep them regardless of stability level.

[jsuereth]

Will update with that justification