Remove `GravityInterface` in favor of `GravityVectorInterface`

[joshuahansel]

Motivation

GravityVectorInterface (in misc module) replaces GravityInterface (in thermal_hydraulics module), so the former should be removed.

Design

• Replace GravityInterface with GravityVectorInterface in apps
• Replace GravityInterface with GravityVectorInterface in THM
• Remove GravityInterface

Impact

Adjust to new APIs. Also GravityInterface uses a different default gravity magnitude, so some tests will need to be adjusted.