Add support for the `price-changes` feature

[remcotolsma]

With https://github.com/pronamic/wp-pronamic-pay-restrict-content-pro/pull/18 we also support the price-changes feature.

• https://plugins.trac.wordpress.org/browser/restrict-content/tags/3.2.10/core/includes/admin/memberships/edit-membership.php#L340
• https://plugins.trac.wordpress.org/browser/restrict-content/tags/3.2.10/core/includes/memberships/class-rcp-membership.php#L3345

Shall we add this to supports array?

https://github.com/pronamic/wp-pronamic-pay-restrict-content-pro/blob/69bdd7b70792f6526cbae5940f14e39a92fd27fb/src/Gateways/Gateway.php#L43-L60

• https://github.com/pronamic/wp-pronamic-pay-restrict-content-pro/issues/17