Update actions/checkout action to v6

| datasource  | package          | from | to |
| ----------- | ---------------- | ---- | -- |
| github-tags | actions/checkout | v5   | v6 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/markdown-linting.yml

@@ -10,7 +10,7 @@ jobs:
   linting:
       runs-on: ubuntu-latest
       steps:
-        - uses: actions/checkout@v5
+        - uses: actions/checkout@v6
         - uses: DavidAnson/markdownlint-cli2-action@v20
           with:
             config: '.markdownlint.json'

.github/workflows/publish.yml

@@ -7,7 +7,7 @@ jobs:
   push-to-live:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           ref: staging
           fetch-depth: 0

.github/workflows/validate-packages.yml

@@ -11,7 +11,7 @@ jobs:
       runs-on: ubuntu-latest
       steps:
         - name: Checkout
-          uses: actions/checkout@v5
+          uses: actions/checkout@v6
 
         - name: schema-validation-action
           uses: cardinalby/[email protected]