[testapp] Update testapp RN-related dependencies to latest

[c0mtru1se]

Task specification

Currently, there are couple of things broken with our RN test app.

Boost was previously hosted on a public JFrog, however it was moved (for various reasons) to a dedicated repo hosted by Boost's org.

React Native core released a series of patches (with a related issue: RN) with the new URLs, however the example app is behind the patch cycle.

As a result, on a fresh machine with non-cached Boost pods now can't compile (neither pod install nor gradle assembling (when not using prebuilds for Android) compile or resolve correctly).

The workaround on non-patched version is to patch-package Boost's podspec, however thats flaky (and unnecessary).

We need to update react-native and all related project files at least to 0.71.15 (easy, short-term fix), but better would be to use latest 0.76+ considering the new-architecture-by-default.