ci: bump actions/checkout to v5

[defangdevs]

actions/checkout@v4 is a Node 20 action; GitHub Actions is deprecating Node 20 runners (https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/). This bumps to v5, which uses Node 24 and removes the deprecation warning.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 743d28f and 4baaf2a.

📒 Files selected for processing (7)

• .github/workflows/build-samples-json.yml
• .github/workflows/check-sample.yml
• .github/workflows/deploy-changed-samples.yml
• .github/workflows/publish-sample-template.yml
• .github/workflows/test-scripts.yml
• .github/workflows/update-template-workflows.yml
• starter-sample/.github/workflows/defang.yaml

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