Deprecate Sponge mechanic with note to use CraftBook 5

**Is your feature request related to a problem? Please describe.**
The sponge mechanic is duplicated across both WorldGuard and CraftBook, with CraftBook having a more feature-complete version. This also doesn't fit into WG as well as it does to CB.

**Describe the solution you'd like**
Deprecate for removal in WG8, with a note to use CB5 instead.

**Describe alternatives you've considered**
Stay as-is I guess

**Additional context**
Ideally, CraftBook5 is in a mostly-complete beta or public release state by the time WG8 rolls around, but it doesn't overally matter.