Merge opentelemetry-api-events into opentelemetry-extension-incubator

[jack-berg]

Writing this made me realize that its silly that we incubate APIs in both opentelemetry-api-events and opentelemetry-extension-incubator. I propose we merge events into opentelemetry-extension-incubator, and do it in lockstep with #6001 to minimize churn.

Originally posted by @jack-berg in #6120 (comment)

[trask]

adding #6120 (comment) here as well:

maybe opentelemetry-extension-incubator -> opentelemetry-api-incubator?

since it's not really an incubator for extensions

(also https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation-api-incubator)

[jack-berg]

maybe opentelemetry-extension-incubator -> opentelemetry-api-incubator?

Yeah that makes sense to me.

[jkwatson]

seems fine to me