Exclude assigned user when checking seats available

What does this MR do and why?

In this MR, we:

• Exclude assigned user when checking seats available
• Assigned user is excluded when calculating seats

Mutation Example:

mutation {
  userAddOnAssignmentBulkCreate(
    input: {userIds: ["gid://gitlab/User/13", "gid://gitlab/User/19", "gid://gitlab/User/20"], addOnPurchaseId: "gid://gitlab/GitlabSubscriptions::AddOnPurchase/6"}
  ) {
    addOnPurchase {
      id
    }
    users {
      edges {
        node {
          id
          name
          username
        }
      }
    }
    errors
  }
}

Response

Related to #451174