ci: use .node-version to maintain node version

[Alfred-Skyblue]

Define Node.js version using environment variables.
https://docs.github.com/en/actions/learn-github-actions/variables

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	85.9 kB	32.7 kB (+8 B)	29.5 kB
vue.global.prod.js	132 kB	49.3 kB (+7 B)	44.3 kB

Usages

Name	Size	Gzip	Brotli
createApp	47.9 kB	18.8 kB	17.2 kB
createSSRApp	50.6 kB	19.9 kB (+2 B)	18.2 kB
defineCustomElement	50.3 kB	19.6 kB (+1 B)	17.9 kB
overall	61.2 kB	23.7 kB (+3 B)	21.6 kB

Maybe not suitable for CI

[haoqunjiang]

I'm now hesitant about this change. Because I recall a previous bugfix in the ecosystem-ci repo: vuejs/ecosystem-ci@09ddbf8
Considering that even minor Node.js releases could bring trouble to our CI, it isn't a good idea to let the Node.js major version silently bump according to a certain schedule that is out of our control.
Because: 1. we need consistency in our CI; 2. silent version bumps could result in subtle bugs we can't easily identify.

[haoqunjiang]

Now that Renovate is configured for this repo, I think we can also use it for automatic Node.js version updates.

Currently, Renovate doesn't detect the node-version string in GitHub Actions by default. But there are 2 possible workarounds:

1. Use regexManagers: Renovate doesn't update NodeJS version used by GitHub actions renovatebot/renovate#7716 (comment)
2. Use a .node-version file in the project root. It is a widely-supported file format with the exception of nvm (but I wouldn't recommend anyone using nvm in most cases anyway, considering its slowness and being tightly coupled to the shell). Renovate also supports it: https://docs.renovatebot.com/node/#file-support

Personally, I prefer the 2nd option. What do you think?

[Alfred-Skyblue]

Agreed, I also think the second approach is indeed better, we can use .node-version file to manage the Node.js version.

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