Update Action configuration to use Node 16.

Author: DavidAnson

action.yml

@@ -2,13 +2,13 @@ name: 'markdownlint-cli2-action'
 author: 'David Anson'
 description: 'A GitHub Action to run the markdownlint-cli2 tool for linting Markdown/CommonMark files with the markdownlint library'
 branding:
-  icon: 'check-square'  
+  icon: 'check-square'
   color: 'orange'
 inputs:
   globs:
     description: 'Glob expression(s) of files to lint (newline-delimited)'
     default: '*.{md,markdown}'
     required: false
 runs:
-  using: 'node12'
+  using: 'node16'
   main: 'dist/index.js'