Update node version to node20 (#55)

* Bump node to 20 in runtime action

* Update README instruction to favor an up-to-date action for checkout

This prevent a warning in Github checks because actions/checkout@v2 is
still using node12 which is deprecated

Author: GregdTd

README.md

@@ -21,7 +21,7 @@ jobs:
     steps:
 
       - name: Checkout repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - uses: quizlet/argocd-diff-action@master
         name: ArgoCD Diff

action.yml

@@ -29,5 +29,5 @@ inputs:
     default: legacy
     required: false
 runs:
-  using: 'node12'
+  using: 'node20'
   main: 'dist/index.js'