Define a process for dealing with OTel SemConv changes

The mapping for OTel data currently supports a certain (old) version of the semantic conventions.

With the OTel Semantic Conventions being merged with ECS and being stabilized we have to expect versions of SemConv to come soon that will introduce many breaking changes in the field names.

We need to define a process for dealing with different versions of Semantic Conventions so that we can support newer versions of SemConv while keeping backwards compatibility with older versions. The implication is that, with a given version of apm-data we should support a range of SemConv versions.

## Related Info
- Semantic conventions define [schema files](https://github.com/open-telemetry/semantic-conventions/tree/main/schemas) that enumerate all the changes between versions
- the semantic conventions versions theoretically may even vary per signal (within a single connection / agent / SDK)