ci: upgrade actions/checkout from v2 to v4 in knative-downstream workflow

[Ankitsinghsisodya]

Fixes #8987

Summary

• Replace actions/checkout@v2 with actions/checkout@v4 on both checkout steps in .github/workflows/knative-downstream.yaml

actions/checkout@v2 is deprecated and no longer receives security patches. v4 is the current stable release.

Test plan

• Workflow syntax is valid YAML
• Downstream CI jobs run successfully with updated action

[knative-prow]

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This pull-request has been approved by: Ankitsinghsisodya
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[knative-prow]

Welcome @Ankitsinghsisodya! It looks like this is your first PR to knative/eventing 🎉

[knative-prow]

Hi @Ankitsinghsisodya. Thanks for your PR.

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[Copilot]

Pull request overview

Updates the Knative downstream GitHub Actions workflow to use a supported and security-patched version of the actions/checkout action.

Changes:

• Upgrade actions/checkout from v2 to v4 for the upstream checkout step
• Upgrade actions/checkout from v2 to v4 for the downstream checkout step

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