Use effective strain rate in viscoplastic stress limiter case as well

[anne-glerum]

In the viscoplastic material model, the strain rate can adapted for elastic stresses and is then called effective_edot_ii instead of edot_ii. effective_edot_ii is then used to compute the current cohesion and friction angle and the resulting yield stress. This yield stress is used in the stress limiter case, which however uses edot_ii in further calculations. This PR changes edot_ii to effective_edot_ii in the stress limiter case to be consistent.

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[tjhei]

No test is affected by this. Is this expected?

[anne-glerum]

There is only one test that uses the limiter, and it doesn't use elasticity. In this case edot_ii = effective_edot_ii. So yes, it's expected.

[bobmyhill]

Thanks for the fix @anne-glerum . You commented just before I could press send!

I'll merge this now, as this fix is independent of any testing that we should do (#5983), and I already suggested some potential tests in a reply to a question from @naliboff on Element.