Problem with access to MySql database

I am learning programming and I am struggling with a problem in my web application. I have made the same application using SQLServer and I have not had problems but changing the connection string to make it work with MySql stops working.

I am trying to validate a login using a user query and password in the database through a Select statement. The database created in Mysql is identical to the one I made in SQLServer (in fact I have tried the query and there are no problems). The only error that I launched was with the connector that sent me this message: "Loading class com.mysql.jdbc.Driver'. This is deprecated. The new driver class is com.mysql.cj.jdbc.Driver '" and that was solved by adding that "cj". The rest is completely the same.

I leave my classes implemented and I thank you in advance for any help you can give me. by the way I use Netbeans IDE 8.2, Mysql 8.0 and this connector "mysql-connector-java-8.0.12.jar"

connection class:

package conexion;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class Conexion {

private static Conexion instancia=null;
private static Connection con=null;
private static final String url="jdbc:mysql://localhost:3306/mantenimiento?useTimezone=true&serverTimezone=UTC";
private static final String driver="com.mysql.cj.jdbc.Driver";
private static final String usuario="sa";
private static final String clave="123456";

public Conexion() {

    try {
        Class.forName(driver);
        con=DriverManager.getConnection(url, usuario, clave);
        System.out.println("Conectado");
    } catch (ClassNotFoundException | SQLException e) {
        System.out.println(e.getMessage());
    }    
}

public synchronized static Conexion conectar(){
if(instancia==null){
  instancia = new Conexion();        
}
return instancia;
}

public Connection getCon() {
    return con;
}

public void cerraConexion(){    
    instancia=null;
}

User class:

package dtos;


public class Usuario {
private String correo;
private String clave;

public Usuario(String correo, String clave) {
    this.correo = correo;
    this.clave = clave;
}

public String getCorreo() {
    return correo;
}

public void setCorreo(String correo) {
    this.correo = correo;
}

public String getClave() {
    return clave;
}

public void setClave(String clave) {
    this.clave = clave;
}

}

LoginDAO class;

package daos;

import conexion.Conexion;
import dtos.Usuario;
import java.sql.PreparedStatement;
import java.sql.ResultSet;


public class LoginDAO {
private static final String sql_select="select * from usuario where correo=? and clave=?";
private static PreparedStatement pstm=null;
private static ResultSet res = null;
private static Conexion con = Conexion.conectar();

public boolean validarLogin(Usuario usuario){
    boolean resultado=false;
    try {

        pstm = con.getCon().prepareStatement(sql_select);
        pstm.setString(1,usuario.getCorreo());
        pstm.setString(2,usuario.getClave());
        res =pstm.executeQuery();

        if(res.next()){
         resultado=true;
        }      
    } catch (Exception e) {
        System.out.println("Error"+e.getMessage());
    }
    finally{
        try {
            if(res!=null)res.close();
            if(pstm!=null)pstm.close();
            if(con!=null)con.cerraConexion();
        } catch (Exception e) {
            System.out.println("Error :"+e.getMessage());
        }
    }

   return resultado; 

}

}

Servlet controller:

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    String correo = request.getParameter("txtCorreo");
    String clave = request.getParameter("txtClave");

    Usuario usuario = new Usuario(correo, clave);
    LoginDAO login  = new LoginDAO();

    if (login.validarLogin(usuario)) {
        request.getSession().setAttribute("correo",usuario.getCorreo());
        request.getRequestDispatcher("sistema.jsp").forward(request, response);

    } else {
       String error = "Usuario y/o contraseña incorrecto"; 
       request.getSession().setAttribute("error",error);
       request.getRequestDispatcher("error.jsp").forward(request, response); 

    }
}