Declare OTLP stable

Since OTLP/JSON was declared stable, all the parts of the OTLP spec is stable
so the whole spec should be declared as such
See gh-2930

Signed-off-by: Jonatan Ivanov <jonatan.ivanov@gmail.com>

specification/protocol/otlp.md

@@ -1,6 +1,6 @@
 # OpenTelemetry Protocol Specification
 
-**Status**: [Mixed](../document-status.md)
+**Status**: [Stable](../document-status.md)
 
 The OpenTelemetry Protocol (OTLP) specification describes the encoding, transport,
 and delivery mechanism of telemetry data between telemetry sources, intermediate
@@ -52,17 +52,6 @@ nodes such as collectors and telemetry backends.
 OTLP is a general-purpose telemetry data delivery protocol designed in the scope
 of the OpenTelemetry project.
 
-## Signals Maturity Level
-
-Each signal has different support and stability in OTLP, described through its
-own maturity level, which in turn applies to **all** the OTLP Transports listed below.
-
-* Tracing: **Stable**
-* Metrics: **Stable**
-* Logs: **Stable**
-
-See [OTLP Maturity Level](https://github.com/open-telemetry/opentelemetry-proto#maturity-level).
-
 ## Protocol Details
 
 OTLP defines the encoding of telemetry data and the protocol used to exchange
@@ -84,8 +73,6 @@ All server components MUST support the following transport compression options:
 
 ### OTLP/gRPC
 
-**Status**: [Stable](../document-status.md)
-
 After establishing the underlying gRPC transport, the client starts sending
 telemetry data using unary requests using
 [Export*ServiceRequest](https://github.com/open-telemetry/opentelemetry-proto)
@@ -391,8 +378,6 @@ connection cannot be established.
 
 #### Binary Protobuf Encoding
 
-**Status**: [Stable](../document-status.md)
-
 Binary Protobuf encoded payloads use proto3
 [encoding standard](https://developers.google.com/protocol-buffers/docs/encoding).
 
@@ -401,8 +386,6 @@ request and response headers when sending binary Protobuf encoded payload.
 
 #### JSON Protobuf Encoding
 
-**Status**: [Stable](../document-status.md)
-
 JSON Protobuf encoded payloads use proto3 standard defined
 [JSON Mapping](https://developers.google.com/protocol-buffers/docs/proto3#json)
 for mapping between Protobuf and JSON, with the following deviations from that mapping: