dependabot: ignore `groot`

Created by: c-dilks

We recently discovered that groot version numbers are out of order. https://github.com/JeffersonLab/coatjava/pull/218 was actually a regression by several versions; therefore we revert by https://github.com/JeffersonLab/coatjava/pull/261.

This PR tells dependebot to ignore groot to prevent this from happening again; updates to groot will henceforth be manual.