refactor: Add users_to_add and users_to_remove fields to replace user update mode

[fregataa]

partial-fix #1697

1. ModifyGroup mutation has been used to add users to group and delete users from group.
   Let's refactor the mutation.

2. Revert some description/deprecation_reason string generator functions for good readability

Test query

mutation ModifyGroup {
    modify_group(
        gid: "GROUP_ID",
        props: {
            add_users: [],
            remove_users: ["USER_ID"],
        }
    ) {
        ok
        msg
        group {
            name
        }
    }
}

Checklist: (if applicable)

• Milestone metadata specifying the target backport version

[achimnol]

Since this is an active API operation, we should use the active voice for the field names.

Let's rename:

• added_users → add_users
• removed_users → remove_users

[achimnol]

Other parts looks good. Just a minor update please.

[agatha197]

LGTM