Refactor: change the name of keyword force_thr_ev2 to force_zero_out for clarity

[kirk0830]

As mentioned in issue #6304 , the keyword force_thr_ev2 is somehow confusing and misleading. After discussion, I suggest to change the name to force_zero_out to reflect its real functionality

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #6304

What's changed?

• Change the input parameter name of force_thr_ev2 to force_zero_out for clarity