[Customer] Deprecate group_id in Customer

[paliarush]

Customer group should not be exposed in the storefront scenarios.

Acceptance criteria:

• Deprecate group_id in Customer type
• Return null instead of actual group ID for all queries.

[fooman]

Why remove it?

There is lots of reasons one might want to customise the front-end based on customer group. For example you might get to see a different banner, description, etc.

Removing group_id from graphql without a replacement makes building such functionality harder. Not everything is between logged in vs logged out either.