Switch plan type to Paid by default, allow price to be optional

[brianhogg]

Description

New access plans are now "paid" by default as they were prior to 7.8

Fixes #2794 Refs https://github.com/gocodebox/lifterlms-integration-woocommerce/issues/176

How has this been tested?

Manually

Checklist:

• [ ] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.