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