Declare logs Stable (#376)

The log data model is now declared Stable: open-telemetry/opentelemetry-specification#2387

The OTLP proto in this repository now matches the log data model
in the specification.

This PR declares OTLP proto for logs Stable as well.

README.md

@@ -32,7 +32,7 @@ metrics/\*<br>collector/metrics/*    | Stable   |
 resource/*                           | Stable   |
 trace/trace.proto<br>collector/trace/* | Stable   |
 trace/trace_config.proto             | Alpha    |
-logs/\*<br>collector/logs/*          | Beta     |
+logs/\*<br>collector/logs/*          | Stable   |
 **JSON encoding**                    |          |
 All messages                         | Alpha    |