chore(testapp): bump RNTA to latest 3.x

[kelset]

Overview

Hey team 👋

A few weeks back I noticed this PR #583 and it made me realise that we (RNTA authors) haven't created proper docs for how to upgrade both the RN version and the RNTA version of an existing RNTA-based test app 😱 sorry about that! (I'll get to that soon!)

So I took a bit of time today to bump this repo's example app to latest RNTA.
It's actually pretty simple:

• you change the version in the package.json of the example app (you don't need the one in the root level, so I removed it - yes, it's another thing that we can tweak in the docs to make clearer)
• you run yarn configure-test-app --force; it will basically update and realign the files so that it will be as-if you just generated the example app with latest RNTA
• you verify the changes and remove anything extra that shouldn't be there

And this PR is the result of that; as you can see with RNTA 2.0 -> 3.0 some files got nicely cleaned up, like the metro config and the react-native config (less things to keep track of!)

Merging this PR should also mean that #477 can be closed (cc @Naturalclar)

Test Plan

The commands for testing the test app work just fine; so

yarn bootstrap
yarn example start
yarn example android
yarn example ios

all work.

Here's a (not that meaningful since well the whole point was that nothing should change) screenshot comparison:

Before (RNTA 2)

After (RNTA 3)

[Naturalclar]

thank you so much! ill try to apply these changes to other repositories as well

[kelset]

(side update: we added a dedicated page in the wiki https://github.com/microsoft/react-native-test-app/wiki/Upgrading )