Update the OAuth service URL

[dkotter]

Description of the Change

This PR updates the URL that we use for our OAuth service

How to test the Change

Either in a new environment or in an existing environment where you've disconnected your Mailchimp account, run through the OAuth flow and ensure it connects correctly and all API requests work as expected.

Changelog Entry

n/a as the changelog from #47 is sufficient

Credits

Props @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

Note that in my testing, this new service works as expected but there are still branding changes that need made.

• If you go straight to the service, it still mentions WooCommerce and shows the Woo logo: https://wordpress.mailchimpapp.com/

• The app on the Mailchimp side doesn't show a logo at all and shows WordPress with a lower case p:

[dkotter]

Following up on the above, found out that the Mailchimp app is created under the 10up account so I was able to update the text and logo, so that part looks better now: