A new, upgraded style of soil sensor 🪴💧🌡️

For a while, we've had a SAMD09-based soil sensor (https://www.adafruit.com/product/4026) that uses the built-in capacitive sensing. It works well but has some non-linearity in its response to wet soil. We wanted to try a different system of measuring capacitance and update it to a 3~5V friendly ATtiny816 (https://www.adafruit.com/product/5681), which lets us ditch the regulator and shifting circuitry.