Update checkout action version to v6

Signed-off-by: Scott Kingsley Clark <lol@scottkclark.com>

Author: sc0ttkclark

.github/workflows/tests-plugin-checker.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
 
       - name: Run plugin check
         uses: wordpress/plugin-check-action@v1