chore(ci): upgrade actions/checkout to v6 (#259)

GitHub removes Node.js 20 from hosted runners in Sept 2026 (force-default
to Node 24 on June 2, 2026). actions/checkout@v4 and older majors run on
Node 20 and will stop working. @v6 runs on Node 24 and adds credential-file
isolation.

Part of a workspace-wide sweep across 15 Pendle repos.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: manh-pendle

.github/workflows/deploy.yaml

@@ -11,7 +11,7 @@ jobs:
     name: Deploy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-node@v4
         with:
           node-version: 20 # Use a recommended Node.js version (Docusaurus requires >= 18)