Add missing base class call in JoltArea3D::post_step

[mihe]

Fixes #101695.

We need to call the non-empty base class implementation of JoltShapedObject3D::post_step from JoltArea3D::post_step, otherwise we never clear JoltShapedObject3D::previous_jolt_shape after modifying the Area3D shape(s), which causes the forced exit + enter events to be queued up in JoltContactListener3D::_flush_area_shifts every single physics step, as opposed to just once.

[mihe]

I'm realizing that #101695 is tangentially related to #101721, so I'll be putting up a new pull request that fixes both of them, where this JoltArea3D::post_step method will go away completely.

[mihe]

The superseding PR can be found here: #101815