[13.x] Generate plural morph pivot table name

[willrowe]

This is another attempt to fix #43800. In the documentation, polymorphic relationships use pivot tables with plural names. However, the Pivot model class does not take that into account when automatically generating the table name since standard (non-polymorphic) pivot tables use singular names and the MorphPivot class extends from it.

The tests never properly covered this case since all usages of the MorphPivot class set the table name explicitly since it does not match the class names.

This is a breaking change because existing code could be depending on the morph pivot table name being singular in order to work around the bug.

[willrowe]

The failing tests seem to be due to some unrelated deprecation notices.