Cannot access a private group's issue board when not a member of the group

Summary

Regression discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/54042#note_144234833

Zendesk Ticket: https://gitlab.zendesk.com/agent/tickets/114275

When a user is not a member of a private group but is a member of some projects under that group, we give access to the group page with issues scoped only to the projects the user has access to.

After a security release a few weeks ago, the issue boards page for this case no longer works.

The /-/boards/1/lists endpoint returns a 403

CC @J-roen