upgrade to Node 16

Running actions on Node 12 has been deprecated [1] and causes a
warning when running workflows with this action.

[1] https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

Signed-off-by: Matthias Aßhauer <mha1993@live.de>

action.yml

@@ -32,5 +32,5 @@ outputs:
   issues:
     description: "issue IDs, comma separated"
 runs:
-  using: "node12"
+  using: "node16"
   main: "dist/index.js"