chore!: drop support of NPM 5

[haoqunjiang]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

[fangbinwei]

engines field of @vue/cli, @vue/cli-service also needs to be modified? And the original npm version of Node.js 10.16.0 is 6.9.0.

vue-cli/packages/@vue/cli/package.json

Lines 64 to 66 in ba6cb7c

	"engines": {
	"node": ">=8.9"
	}

I am testing @babel/eslint-parser and eslint7 recently, and eslint7 requires nodejs ^10.12.0, or >=12.0.0. Maybe it should be considered ?

[haoqunjiang]

Yeah right, I'll bump it in a separate PR.