Check if WooCommerce is older than 7.3 in `wcs_get_page_screen_id()`.

[shendy-a8c]

Fixes #358

Description

We want wcs_get_page_screen_id( 'shop_subscription' ) to return 'shop_subscription' and since wc_get_page_screen_id() will do so since WC 7.3 (at least it still returns an empty string in 7.2.2), we should check wcs_is_woocommerce_pre( '7.3.0' ) instead of 6.9.0.

ps: I don't include changelog because this bug has not been released yet to public.

How to test this PR

1. Install WC < 7.2.2.
2. Create a new product (/wp-admin/post-new.php?post_type=product).
3. You should not see a fatal error.

Product impact

• [x] Added changelog entry (or does not apply)
• [x] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [x] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [x] Added deprecated functions, hooks or classes to the spreadsheet

[james-allan]

This looks good to me.

As the original author of the function wcs_get_page_screen_id() I was curious why the 6.9.0 check was failing. It turns out that although the function was introduced in 6.9.0, it wasn't until 7.3.0 that it could actually handle screen IDs other than shop_order.

For reference, here's the WC core PR https://github.com/woocommerce/woocommerce/pull/35658 and a link to specific changes.