Update resource model in versioning and stability specification.

[jsuereth]

We need to update: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/versioning-and-stability.md#semantic-conventions-stability

Right now this calls out which fields are considered "stable" for semantic conventions. However, the way resource is called out, and what semantic conventions can actually enforce are problematic.

With the addition of "entities" we'll be able to enforce stability on appropriate components within the SDK, we need to update the specification here once the data model specification is laid out.

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