CI: Update checkstyle checkout action to v6

The checkstyle workflow was the only one still pinned to
actions/checkout@v4; the other workflows already use v6.
Bump it to match.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Christos Longros <[email protected]>
Closes #18600

.github/workflows/checkstyle.yaml

@@ -12,7 +12,7 @@ jobs:
   checkstyle:
     runs-on: ubuntu-22.04
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         ref: ${{ github.event.pull_request.head.sha }}
     - name: Install dependencies