Rename module to refer to the react checkout

[Vinai]

Now that Hyvä Checkout has been released, the module name is misleading.
A better name would be something like hyva-themes/magento2-react-checkout-braintree. This should also be clarified in the readme.

[rajeev-k-tomy]

This is something @wigman can do. After changing the name, I will update the documentation