MM-31054 demote user (#16990)

* changing SchemeAdmin to false on demotion

* Added test

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/user_store.go

@@ -1884,6 +1884,7 @@ func (us SqlUserStore) DemoteUserToGuest(userID string) (*model.User, error) {
 
 	query = us.getQueryBuilder().Update("ChannelMembers").
 		Set("SchemeUser", false).
+		Set("SchemeAdmin", false).
 		Set("SchemeGuest", true).
 		Where(sq.Eq{"UserId": userID})
 
@@ -1898,6 +1899,7 @@ func (us SqlUserStore) DemoteUserToGuest(userID string) (*model.User, error) {
 
 	query = us.getQueryBuilder().Update("TeamMembers").
 		Set("SchemeUser", false).
+		Set("SchemeAdmin", false).
 		Set("SchemeGuest", true).
 		Where(sq.Eq{"UserId": userID})