Update dependencies, upgrade unified-ci

[rvagg]

Replaces #366

upgraded dependencies in v0, v2 and cmd

Also added a dependabot conifg, the only catch with that is I don't know if I should use /cmd/car or /cmd. The go.mod is in /cmd, but you can't run go get from in there. If nobody else knows what's the right way to do that then we can proceed as is and fix later if it doesn't update properly.

[rvagg]

The macos CI failure is from rogpeppe/go-internal#200, which is still unsolved. We're either going to have to live with it, migrate to a new tool, or figure out how to get unified-ci to downgrade our macos version in the OS matrix.

[rvagg]

hat tip to @laurentsenta for the hot tip that we can use UCI_GO_TEST_RUNNER_MACOS as a repo variable in here to downgrade the macos runner to "macos-11" so go-internal doesn't keep on failing on us while rogpeppe/go-internal#200 remains unresolved.

The trick now will be remembering that this is set so that when GitHub retires macos-11 we don't have to scratch our heads why we're still using it here; cause it's kind of tucked away.