Update the extension namespace to SolidusBraintree

[gsmendoza]

Summary

We want to rename the extension namespace to SolidusBraintree so that hopefully we would have an easier time to merge this repo's code to https://github.com/solidusio/solidus_braintree, which will be done in solidusio/solidus_braintree#92.

Resolves solidusio/solidus_braintree#96 .

Testing

In my manual testing, I was able to

1. Migrate a Rails app with the old extension name (SolidusPaypalBraintree) to the new one.
2. Check out a cart item with the renamed Solidus Braintree extension.

See https://github.com/gsmendoza/solidus_store/tree/rails-7-0-4--solidus-3-2-5--braintree--renamed for the test app.

Reproducing the manual test

1. Set BRAINTREE_MERCHANT_ID, BRAINTREE_PUBLIC_KEY, and BRAINTREE_PRIVATE_KEY environment variables to your Braintree credentials.
2. Clone https://github.com/gsmendoza/solidus_store. Check out rails-7-0-4--solidus-3-2-5--braintree--renamed branch.
3. Run rails db:setup.
4. Run rails spree_sample:load.
5. Run rails server.
6. In Admin > Braintree Configurations, check Credit Card.
7. In Admin > Settings > Payments > Payment Methods, create new Braintree payment method.
8. Update the preference source of the Braintree payment method to braintree_credentials.
9. You should now be able to successfully check out an item from your cart with the new Braintree payment method.

Known issues

The extension doesn't work with SolidusStarterFrontend. That will be fixed in https://github.com/solidusio/solidus_braintree/issues/91

Checklist

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[gsmendoza]

I'll close this PR since I'll be reimplementing this in https://github.com/solidusio/solidus_braintree after https://github.com/solidusio/solidus_braintree/issues/92.