Stabilize opentelemetry-exporter-sender-jdk

[jack-berg]

The JdkHttpSender was added in #5557, July 2023. I know its been used in production systems for some time now, and we've fixed a variety of small bugs that were reported about it.

Let's stabilize it. The API is not going to change (there is no public API surface area), so stabilization is just an indication that we think its ready for production use.