OSS site node 16 upgrade

[jpvajda]

Relates to https://github.com/newrelic/gatsby-theme-newrelic/issues/462

On Wednesday, September 29, Gatsby will be updating the default version of Node.js in Gatsby Cloud to v14. We'll want to upgrade our sites to node 16 soon after so we can get ready for Gatsby 4.

Acceptance Criteria

• [ ] Depends on newrelic/gatsby-theme-newrelic#495
• [x] Confirm with Gatsby that Gatsby 4 supports node 16 - @jpvajda
• [x] upgrade site to Node 16
• [x] upgrade all workflows to Node 16
• [x] update .nvrmc file to node 16
• [x] Confirm if we need to set the NODE_VERSION environment variable
• [x] Ensure all documentation is updated.

[aswanson-nr]

Node 16 goes LTS on the 26th of October. I'd like to propose moving to node 16 instead, that way we're up to date with the Node ecosystem.