Specify Node.js 16.x runtime for action execution in runner

The Node.js version for use when executing the action in the GitHub Actions runner is configured via the `runs.using`
field of the `action.yml` metadata file.

Previously, the action was configured to use Node.js 12.x. This was actually the result of an oversight, as Node.js 16.x
has been used for the development and validation of the action since 2022-01-10. It will be important to use the same
version on the runner as the action is validated for by the project infrastructure in order to ensure the expected
behavior.

In addition, GitHub has deprecated the use of Node.js 12.x runtime. A warning about this was shown in the workflow run
summary page of all workflows using this action. That warning will be resolved, and the eventual complete breakage of
those workflows avoided, by this change.

Author: per1234

action.yml

@@ -40,7 +40,7 @@ inputs:
     required: false
     default: ${{ github.token }}
 runs:
-  using: "node12"
+  using: "node16"
   main: "dist/index.js"
 branding:
   icon: "check-square"