Bump node-version to 16

[ewels]

Fixes markdown linting, which now requires at least node version 12.

Hopefully pushing to 16 gives us some future-proofing for a while.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[ewels]

Just realised that the website doesn't specify the version at all: https://github.com/nf-core/nf-co.re/blob/master/.github/workflows/markdown.yml

Maybe that's the best approach here..

[mashehu]

Nope. Now also the website fails: https://github.com/mashehu/nf-co.re/runs/5106165522?check_suite_focus=true

[ewels]

Seems ok with v2 of the get-node thing though right?

[ewels]

ok, bumped the install node version and stripped the pinned version. Hopefully this works...