[chore] Update README.md to correctly reflect OTLP Protocol version (#…

…11949)

https://github.com/open-telemetry/opentelemetry-collector/blob/main/Makefile#L174
shows v1.4.0, so the readme got stale?

README.md

@@ -97,7 +97,7 @@ calls and don't want them to feel excluded.
 
 ## Supported OTLP version
 
-This code base is currently built against using OTLP protocol v1.3.1,
+This code base is currently built against using OTLP protocol v1.4.0,
 considered Stable. [See the OpenTelemetry Protocol Stability
 definition
 here.](https://github.com/open-telemetry/opentelemetry-proto?tab=readme-ov-file#stability-definition)