Add pmpro-subscription-check to deprecated add ons

[ipokkel]

Is your feature request related to a problem? Please describe.
When The Subscription Check Add On is activated is triggers fatal errors as it is looking to use an undefined method getCustomer from the core include Stripe library.

Describe the solution you'd like
Subscription check abilities are now incorporated into core (3.0+) and a notification to uninstall the deprecated Subscription Check Add On would be helpful.