Upgrade to JSpecify and OkHttp 5.+

[magicprinc]

JSpecify is de-facto Java Null-Safe standard (Google, IDEA, Spring all support it)

OkHttp 5.+ uses Lock vs synchronized and is VirtualThread safe

🙏

[JakeWharton]

We have already landed OkHttp 5. You're free to use it in your builds with any version of Retrofit 2.x or 3.x in the mean time.

I doubt we'll migrate nullness annotations at this point. Any effort here is probably better spent migrating to Kotlin.