[connector/otlpjson] Move stability to alpha

[ChrsMark]

Description:

This is the 3rd PR for the new otlpjson connector (#34208). Moves its stability to alpha.

Link to tracking Issue: #34208 #34239 #34249

Testing: ~

Documentation: Updated

[songy23]

Please update https://github.com/open-telemetry/opentelemetry-collector-releases/blob/main/distributions/otelcol-contrib/manifest.yaml too

[djaglowski]

I assumed the config.go code was a placeholder, but if we don't need any custom validation I believe we can delete it.

[ChrsMark]

I assumed the config.go code was a placeholder, but if we don't need any custom validation I believe we can delete it.

👍🏽 . As proposed at #34249 (comment) for now there is no exposed config and we can go with plain pass-through approach. We can iterate on this if we have specific requests/use-cases.
I have removed the Validate().