Use OTel semconv package instead of consts

The telemetry package should probably use the OTel semconv package for the consts below. The benefit of doing so is that updates to the semconv would generally require minimal changes to the telemetry package. Since the constants are already defined there, it does not make sense to redefine them in the telemetry package.

See the consts defined under the "feature_flag" namespace: https://pkg.go.dev/go.opentelemetry.io/otel@v1.37.0/semconv/v1.34.0#section-readme:~:text=Namespace%3A%20feature_flag

https://github.com/open-feature/go-sdk/blob/d7174f73db903064b8170007c2a347b727379cab/openfeature/telemetry/telemetry.go#L27-L36

Start in openfeature/telemetry/telemetry.go, especially the constants referenced around lines 27–36, and compare them with the OpenTelemetry semconv feature_flag namespace. Replace the duplicated definitions with the corresponding semconv constants, then verify that the telemetry package still builds and its existing behavior is unchanged.

Written by the indexing model from the issue text.