chore: update node 8 to 16

[magdziarek]

• [X] Ready for review
• [X] Follows CONTRIBUTING rules

What does this PR do?

In preparation for the Gradle plugin improvements this:

• updates node version from 8 to 16 😨
• updates documentation including changing formatting of the README file (result of running npm run lint)

How should this be manually tested?

1. Clone and test the following Gradle projects with snyk CLI: vuln project 1, vuln project 2, non-vuln Android project, non-vuln project
2. Check the CLI returns the same results for the above projects when using this branch of snyk-gradle-plugin. To do this:
   • Pull this branch of snyk-gradle-plugin
   • Clone snyk/cli
   • On the master branch, in the CLI package.json change "snyk-gradle-plugin": "3.17.0" to "snyk-gradle-plugin": "../snyk-gradle-plugin"
   • Run: npm i && npm run build
   • Use CLI locally by adding an alias snyk-local='SNYK_API="https://app.dev.snyk.io/api/v1" node /Users/XXXX/cli/dist/cli' -> pointing to your local CLI repo
   • Run snyk-local test with: vuln project 1, vuln project 2, non-vuln Android project, non-vuln project

What are the relevant tickets?

TARDIS-830

[CLAassistant]

All committers have signed the CLA.