Fix depreciation of set_design_var

[dzalkind]

Purpose

Fix OpenMDAO 3.45.0 compatibility: replace deprecated set_design_var with _set_design_var in NSGA2Driver.

The public set_design_var method was deprecated in OpenMDAO 3.40.0 and set to expire in version 3.45.0 (which is now in use). OpenMDAO's guidance directs driver subclasses to use the private _set_design_var instead, which is the sanctioned internal API for optimizers setting design variables in driver-scaled space.

This broke:

• The iea22_raft_opt_driver.py example in WEIS (see WEIS CI job)
• 4 of 6 tests in test_nsga2_driver.py

All built-in OpenMDAO drivers (pyoptsparse, pymoo, scipy, modopt) already use _set_design_vars / _set_design_var internally, so this aligns NSGA2Driver with the framework's current pattern.

Type of change

• Bugfix (non-breaking change which fixes an issue)

Testing

All 6 tests in test_nsga2_driver.py now pass.

Checklist

• [x ] I have run existing tests which pass locally with my changes
• I have added new tests or examples that prove my fix is effective or that my feature works
• I have added necessary documentation

[cfrontin]

thanks for the fix!