Update Node.js from v12 to v16

According to the release policy of Node.js,
Node.js v12 reached EOL (end-of-life).

https://nodejs.org/en/about/releases/

I think ruby/setup-ruby is ready to update Node.js to v16
because GitHub Actions runners support v16.

https://github.com/actions/runner/issues/772

.tool-versions

@@ -1,2 +1,2 @@
-nodejs 12.0.0
+nodejs 16.0.0
 ruby 2.7.0

README.md

@@ -35,13 +35,13 @@ which means Ruby ≤ 2.4 is unmaintained and considered insecure.
 
 ### Supported Platforms
 
-The action works for all [GitHub-hosted runners](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners).
+The action works on these [GitHub-hosted runners](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners) virtual environments. Virtual environments not listed below are unsupported.
 
 | Operating System | Recommended | Other Supported Versions |
 | ----------- | -------- | -------- |
 | Ubuntu  | `ubuntu-latest`  (= `ubuntu-20.04`) | `ubuntu-18.04` |
 | macOS   | `macos-latest`   (= `macos-10.15`)  | `macos-11.0` |
-| Windows | `windows-latest` (= `windows-2019`) | `windows-2022` |
+| Windows | `windows-latest` (= `windows-2022`) | `windows-2019` |
 
 The prebuilt releases are generated by [ruby-builder](https://github.com/ruby/ruby-builder)
 and on Windows by [RubyInstaller2](https://github.com/oneclick/rubyinstaller2).

action.yml

@@ -32,5 +32,5 @@ outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'
 runs:
-  using: 'node12'
+  using: 'node16'
   main: 'dist/index.js'