RFC 167: Not allowed to apply license with unknown plan

Created by: unknwon

New licenses should not be able to be used in old Sourcegraph instances, and new Sourcegraph instances should not use a license with old/unknown tags.

Most likely adding code here, when it looks like a plan but we do not recognize it, we reject to apply the license: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/enterprise/cmd/frontend/internal/licensing/plans.go#L39

Part of RFC 167.