Json payload in Kafka exporter is not experimental (#23884)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->

**Link to tracking Issue:** <Issue number if applicable>

Resolves
#23871

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

Signed-off-by: Pavol Loffay <[email protected]>

exporter/kafkaexporter/README.md

@@ -26,7 +26,7 @@ The following settings can be optionally configured:
 - `topic` (default = otlp_spans for traces, otlp_metrics for metrics, otlp_logs for logs): The name of the kafka topic to export to.
 - `encoding` (default = otlp_proto): The encoding of the traces sent to kafka. All available encodings:
   - `otlp_proto`: payload is Protobuf serialized from `ExportTraceServiceRequest` if set as a traces exporter or `ExportMetricsServiceRequest` for metrics or `ExportLogsServiceRequest` for logs.
-  - `otlp_json`:  ** EXPERIMENTAL ** payload is JSON serialized from `ExportTraceServiceRequest` if set as a traces exporter or `ExportMetricsServiceRequest` for metrics or `ExportLogsServiceRequest` for logs. 
+  - `otlp_json`:  payload is JSON serialized from `ExportTraceServiceRequest` if set as a traces exporter or `ExportMetricsServiceRequest` for metrics or `ExportLogsServiceRequest` for logs. 
   - The following encodings are valid *only* for **traces**.
     - `jaeger_proto`: the payload is serialized to a single Jaeger proto `Span`, and keyed by TraceID.
     - `jaeger_json`: the payload is serialized to a single Jaeger JSON Span using `jsonpb`, and keyed by TraceID.\