Until now, we have stuck with the 3.12 long-term branch so as
to keep support for older API levels. However, this means that
the version we're using is 2.5 years old at this point. The
3.12 branch will stop receiving critical updates in December
2021.
Since use of the OkHttp extension is optional anyway, it seems
preferable to move to a modern version of OkHttp at this point.
PiperOrigin-RevId: 381465269
