Upgrade from 'node16' to 'node20'

Upgrade the Node.js version used to run this action from Node.js v16 ('node16') to Node.js v20 ('node20'). For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

action.yml

@@ -10,5 +10,5 @@ branding:
   color: "purple"
 description: run gitleaks on push and pull-request events
 runs:
-  using: "node16"
+  using: "node20"
   main: "dist/index.js"