[SEMANTIC CONVENTION] Deprecated semconv (in the spec) not deprecated (in C++)

[marcalff]

Some semantic conventions are flagged as deprecated in the model.yaml file, for example net.peer.name.

When generating C++ code for it, the generated declaration is missing the @deprecated markup.

This is an issue, because code using deprecated semantic conventions goes unnoticed, as it builds without warnings.

It seems the way to represent the deprecated flag has changed in the generator tool.

The code in the opentelemetry-cpp template uses:

{%- if attribute.deprecated %}

while the same in opentelemetry-java was changed recently to

{%- if (attribute.stability | string())  == "StabilityLevel.DEPRECATED" %}

See java PR open-telemetry/opentelemetry-java#4938