Inconsistent port names in MultibodyPlant

[jwnimmer-tri]

The Doxygen overview for MbP lists its promised output port names, but the following two port names are inconsistent between the docs and the actual framework name of the port:

• The doc figure says body_spatial_velocities which matches the getter function, but actual port name is "spatial_velocities".
• The doc figure says body_spatial_accelerations which matches the getter function, but actual port name is "spatial_accelerations".

As best I can tell, this has been broken since its introduction in #13364.

Tagging +@amcastro-tri as component owner.

Version: Drake v1.29.0.

[jwnimmer-tri]

FYI This tripped me up while implementing free body sleeping.

My gut feeling is that we should change the string names, leaving the getters and docs unchanged. The body_poses port follows the same pattern and uses the body_... prefix consistently.