Use the proper function for the knapsack inequality constraint

[hsadeghidw]

currently, the way the inequality constraint is built is very involved. The code can be simplified significantly by using .add_linear_inequality_constraint.

If the goal is to teach users how to work with slack variables, using a higher-level function is not needed. However, more users can expand this example to an application, if it's simplified.

[vgoliber]

We actually reference this example in training when we discuss auxiliary variables, etc. It's definitely something we need to update in our content, but would be good to keep in sync on this so that we don't change the code before training has a chance to adjust.

[vgoliber]

Training should be fine with these changes after the current participants conclude on Friday, 27 August. Thanks!