Change Kennebec to "Merrymeeting Bay"

[danStich]

This river includes a few notable systems that should be gathered under a more intuitive name. The complex of rivers (Kennebec, Androscoggin, Sebasticook, etc.) are often referred to collectively as "Merrymeeting Bay", so this would be a good start.

[danStich]

addressed in recent data overhauls and updates along with others