build: update checkout action to v5

[rejected-l]

Bumps checkout to v5 for future-proofing against Node 24 runner updates. Requires runner v2.327.1+. Workflows compile the same.

More info: https://github.com/actions/checkout/releases/tag/v5.0.0

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[!IMPORTANT] Update actions/checkout to v5 in workflows for future-proofing against Node 24 runner updates.

• Workflows:
  • Update actions/checkout to v5 in ci.yaml and update-feature-gates.yml for future-proofing against Node 24 runner updates.
  • Requires runner v2.327.1+.
• Behavior:
  • Workflows compile the same with no changes to steps or logic.

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[vercel[bot]]

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