Updating WCS_Related_Order_Store_Cached_CPT_Test tests to be compatible with HPOS

[prettyboymp]

Continuation of #449

Description

This addresses the tests within the WCS_Related_Order_Store_Cached_CPT_Test by using the datastore to manage meta values instead of directly manipulating post_meta.

How to test this PR

1. Follow the testing instructions given in the subscriptions-core Unit Tests.
2. Set the HPOS environmental variable when running the test: $> HPOS=1 ./vendor/bin/phpunit --filter=WCS_Related_Order_Store_Cached_CPT_Test

Note, there are still other tests and source that needs to be updated for HPOS.

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