Handle iOS sample app build break when BoringSSL & Libuv version update

Currently if we modify gRPC-Core's [BoringSSL & Libuv version number](https://github.com/grpc/grpc/blob/master/gRPC-Core.podspec#L171-L172) w/o publishing the pod for the new version, our iOS sample app project will break since the next pod install won't be able to find the updated version from the pod trunk (see example https://github.com/grpc/grpc/pull/27710/). A few action items

- we need to add presubmit build test to kokoro that builds an iOS app pod using the updated version from cocoapod trunk. This helps detect build breaks
- Update iOS release process so that this version bump are in sync w/ cocoapod trunk udpate

Start with gRPC-Core.podspec at the BoringSSL and Libuv version references around lines 171-172, then inspect the Kokoro presubmit configuration and the iOS sample app build path. Verify how an updated version is resolved from CocoaPod trunk and document the release-process synchronization; done means the presubmit catches an unpublished version before the sample app build breaks.

Written by the indexing model from the issue text.