Upgrade GitHub Actions to v4 to fix cache error

[Copilot]

The gh-pages workflow was failing with "Cache service responded with 400" due to deprecated cache implementation in v2 actions.

Changes

• actions/checkout: v2 → v4
• actions/setup-node: v2 → v4
• Node.js runtime: 16 → 20

Both test and deploy jobs updated. v4 actions use current GitHub cache backend; Node.js 20 is latest LTS and compatible with existing >=16 requirement.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Upgrade GitHub Actions to fix cache error in gh-pages workflow</issue_title>
<issue_description>## Problem

The Docusaurus workflow is failing with a cache error:

##[error]Cache service responded with 400

Task

Upgrade all GitHub actions to the latest stable version.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Upgrade GitHub Actions to fix cache error in gh-pages workflow #1543

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Important

Upgrade GitHub Actions to v4 and Node.js to version 20 in gh-pages.yml to fix cache error.

• GitHub Actions:
  • Upgrade actions/checkout from v2 to v4 in gh-pages.yml.
  • Upgrade actions/setup-node from v2 to v4 in gh-pages.yml.
• Node.js:
  • Update Node.js runtime from version 16 to 20 in gh-pages.yml.
• Compatibility:
  • v4 actions use the current GitHub cache backend.
  • Node.js 20 is compatible with existing >=16 requirement.

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1. .github/workflows/gh-pages.yml:15

• Draft comment:
  Upgrade to checkout@v4 is appropriate. Verify that 'fetch-depth: 0' remains valid with v4.
• Reason this comment was not posted:
  Confidence changes required: 0% <= threshold 50% None

2. .github/workflows/gh-pages.yml:18

• Draft comment:
  Updated setup-node to v4 with Node.js 20. Confirm that caching parameters ('cache' and 'cache-dependency-path') are still supported in this version.
• Reason this comment was not posted:
  Confidence changes required: 0% <= threshold 50% None

Workflow ID: wflow_fHq5ZkQvYBCTC74I

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[Copilot]

Pull request overview

This PR upgrades GitHub Actions dependencies to fix a cache error (400 response) in the gh-pages workflow caused by deprecated cache implementation in v2 actions.

• Upgrades actions/checkout and actions/setup-node from v2 to v4
• Updates Node.js runtime from version 16 to 20 (latest LTS)
• Changes applied consistently to both the test and deploy jobs

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