Stabilise SpanNames

[brunobat]

Is your feature request related to a problem? Please describe.

This is a request to stabilise this class:
instrumentation-api-incubator/src/main/java/io/opentelemetry/instrumentation/api/incubator/semconv/util/SpanNames.java

Describe the solution you'd like

Span names should now be stable.

Describe alternatives you've considered

No response

Additional context

Idea comes from quarkusio/quarkus#41521 (comment)

[trask]

This seems reasonable to me. Just need to move it from instrumentation-api-incubator to instrumentation-api.