Move httpconv and netconv to go-contrib

[pellared]

Alternative proposal for #3744

1. Create the httpconv and netconv in https://github.com/open-telemetry/opentelemetry-go-contrib
   1.1. They should be internal and we can possibly use the gotmpl tool to avoid cross-module dependencies (reference: Do not depend on internal packages of different modules #3846)
   1.2. The package would just operate on the "latest" (the one currently supported in go-contrib repo) version of semconv
   1.3. Update https://github.com/open-telemetry/opentelemetry-go-contrib instrumentation libraries to use the new packages
2. Update the semconv generation process
   2.1. Remove semconvkit tool and its usage from Makefile
3. Deprecate the existing semconv/internal packages

This should decouple semconv package from the instrumentation libraries and make the creation of new semconv packages easier.

[pellared]

Instead of point 2 and 3 we just need to stop generating new packages.
The semconvkit tool is still needed for generating files like: https://github.com/open-telemetry/opentelemetry-go/blob/main/semconv/v1.20.0/schema.go