Issue warning if JAX_USE_PJRT_C_API_ON_TPU=false
```
$ JAX_USE_PJRT_C_API_ON_TPU=0 python3 -c "import jax"
/home/skyewm/jax/jax/_src/cloud_tpu_init.py:77: UserWarning: JAX_USE_PJRT_C_API_ON_TPU=0 will no longer be supported in an upcoming future release. Please file an issue at https://github.com/google/jax/issues if you need this setting.
warnings.warn(
```
