Use renovate to auto-update Node versions in package.json files

[rsimha]

We've had to periodically update the engines section of our package.json files each time a new version of Node is released. Sometimes, this breaks Travis builds. And at other times, it breaks tools like amphtml-validator.

See #25161 (comment) for a discussion on this.

I propose that we configure Renovate to keep the engines section in all our package.json files up to date. Here are the docs: https://docs.renovatebot.com/node. I think a support policy of active should give us what we're looking for.

[rsimha]

Tagging @ampproject/wg-infra @ampproject/wg-caching @ampproject/wg-runtime @ampproject/wg-performance in case someone can think of a reason not to do this.

@estherkim since you just fixed this for amphtml and validator, can I assign this to you?

[twifkak]

I suggest lts instead of active. Note that in the future there will be only a two day overlap between the "active" status of v12 and v14, which would give people insufficient time to upgrade. (If I'm reading that right...)

edit: Or maybe ['lts', 'active'] to match what was just deployed in #25208.

[rcebulko]

This seems reasonable, especially for minor and patch upgrades

[rsimha]

I had a déjà vu moment and realized that we've already attempted this before: See #19218. Not sure if it doesn't work, or if it hasn't had to do anything thus far. Will need to dig into this some more.

[rsimha]

And here's the long discussion we had when node version 8 gave way to 10: #19050