Update supported node versions

[jwhitlock]

PR #90 is failing because mocha v9 requires node v12 at a minimum, and node 10.24.1 is installed.

v8 is required in package.json:

taskcluster-lib-scopes/package.json

Lines 12 to 14 in ac24289

	"engines": {
	"node": ">=v8.17.0"
	},

v10 is installed in .taskcluster.yml:

taskcluster-lib-scopes/.taskcluster.yml

Line 26 in ac24289

image: "node:10"

Looking at https://nodejs.org/en/about/releases/, I think v12 should be the minimum version.

See also taskcluster/taskcluster-lib-urls#96.