Stabilize "Environment Variables as Context Propagation Carriers" document

[pellared]

What are you trying to achieve?

Implementation compliant the specification available in a few languages: https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md?plain=1#L253. More:

• Add support in SDK's for Environment Variable Context Propagation in support of Spec Changes #4771

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/env-carriers.md marked as stable

Additional context.

Please leave a comment if there is anything that needs to be addressed before the document is marked as stable.

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[pellared]

CC @open-telemetry/semconv-cicd-approvers

[adrielp]

Thank you for carrying this forward @pellared - we greatly appreciate your work on this!