Mark OpenCensus compatibility spec as stable (open-telemetry#3425)

Fixes
open-telemetry#1175

## Changes

Mark the OpenCensus Compatibility specification as stable. We now have
implementations of the bridges in a few languages:
-
[Java](/~https://github.com/open-telemetry/opentelemetry-java/tree/main/opencensus-shim)
– trace and metrics bridges
-
[Go](/~https://github.com/open-telemetry/opentelemetry-go/tree/main/bridge/opencensus)
– trace and metrics bridges
-
[Python](/~https://github.com/open-telemetry/opentelemetry-python/tree/main/shim/opentelemetry-opencensus-shim)
– trace bridge
- JS – trace bridge WIP by me

Co-authored-by: Carlos Alberto Cortez <calberto.cortez@gmail.com>

specification/compatibility/opencensus.md

@@ -1,6 +1,6 @@
 # OpenCensus Compatibility
 
-**Status**: [Experimental](../document-status.md), Unless otherwise specified.
+**Status**: [Stable](../document-status.md), Unless otherwise specified.
 
 The OpenTelemetry project aims to provide backwards compatibility with the
 [OpenCensus](https://opencensus.io) project in order to ease migration of
@@ -80,8 +80,6 @@ Starting with a library using OpenCensus Instrumentation:
 
 ## Trace Bridge
 
-**Status**: [Experimental, Feature Freeze](../document-status.md)
-
 The trace bridge is provided as a shim layer implementing the
 [OpenCensus Trace API](/~https://github.com/census-instrumentation/opencensus-specs)
 using the OpenTelemetry Trace API. This layer MUST NOT rely on implementation