Update build-ot-firmware.yml

Upgrade checkout action to v4

.github/workflows/build-ot-firmware.yml

@@ -32,7 +32,7 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - id: date
         run: echo "::set-output name=date::$(date +'%Y%m%d')"