ci: update actions/checkout from v4 to v6 in all reusable workflows

actions/checkout v6 was already adopted in headlamp-agent-skills.
This brings the org-level reusable workflows (plugin-ci, plugin-release,
ci-health-check) up to the same version. Affects all plugin repos that
call these shared workflows.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci-health-check.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: local-ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup Node.js
         uses: actions/setup-node@v4

.github/workflows/plugin-ci.yaml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup Node
         uses: actions/setup-node@v4

.github/workflows/plugin-release.yaml

@@ -46,7 +46,7 @@ jobs:
           fi
 
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0