Change create product route to `POST /api/products`

[rachel-carvalho]

WHY are these changes introduced?

We want to perform mutations with a POST instead of a GET.

WHAT is this pull request doing?

• Changed GET /api/products/create to POST /api/products
• Point web/frontend submodule to the commit which updates fetch call https://github.com/Shopify/shopify-frontend-template-react/pull/259
• Added /api/products and /api/products/count to the VerifyCsrfToken middleware's list of excepted paths
• Removed variants field from populateProduct mutation because it's not valid anymore
• Added a check of errors in the response body in addition to HTTP status code because most GraphQL failures still return HTTP 200 ok

https://github.com/Shopify/shopify-app-template-php/assets/446046/a4b1b1dc-ee39-4d00-a18b-bccc79bc81b5

Checklist

Note: once this PR is merged, it becomes a new release for this template.

• [ ] ~I have added/updated tests for this change~ N/A
• [ ] ~I have made changes to the README.md file and other related documentation, if applicable~ N/A