Increase min code version so that we can safely ignore unconvenient android notification structure edge case

Summary: This differential increases minimal code version so that we can safely drop support for 'custom_notification' field in Android notification and thus make code simpler

Test Plan: Since code version below 31 is considered very old, we should expect that nothing happens and application cotinues to work correctly both in debug and production builds

Reviewers: tomek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4732