Upgraded embededd OkHttp library to latest version, 3.14.9

[mformetal]

Is your feature request related to a problem?

Not necessarily a problem, but the app I work in uses Retrofit, which as of creating this PR uses version 3.14.9 of OkHttp: https://github.com/square/retrofit/blob/master/build.gradle#L7.

Describe the solution you'd like

I'd like to see if upgrading grpc-java's version of OkHttp to 3.14.9 causes any breaking changes.

Describe alternatives you've considered

I've excluded the embedded version of OkHttp that grpc-okhttp brings in, but it was requested of me to try and get the library aka grpc-java to upgrade internally.

[sanjaypujare]

@mformetal can you clone and build grpc-java yourself after locally upgrading the okhttp version (from the current 2.7.4) to 3.14.9 and see if causes any breaking changes? You can then share your findings here and might be able to create a PR for this change. See a previous PR #6726 for an example.

[voidzcy]

Duplicate of #1628, #2999, #6119.

No, we don't have a short-term plan for upgrading to okhttp3, it is a different implementation not a simple version bump. Your application shouldn't have a problem depending on both grpc-okhttp and okhttp3. Our usage of okhttp internals is very minimal and ProGuard will strip off unnecessary methods for you. You can see more information in #6119.