deps: Upgrade from NodeJS v12 to v16.

GitHub Actions has deprecated NodeJS 12 Actions, leaving the following
annotation on jobs that use them. NodeJS 16 is supported, so upgrade the
send-message Action to use that.

> Node.js 12 actions are deprecated. For more information see:
> https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

Fixes #70.

send-message/action.yml

@@ -27,5 +27,5 @@ inputs:
     description: 'The content of the message. Maximum message size of 10000 bytes.'
     required: true
 runs:
-  using: 'node12'
+  using: 'node16'
   main: '../dist/send-message/index.js'