docs: require Node 20.9.0 for release/3.12

[Federicorao]

Summary

• update the release/3.12 README requirement from Node 18+ to Node 20.9.0+
• align the root package.json engines.node value with the documented requirement and existing migration guide

Verification

• node -e check confirmed package.json and README.md both require Node 20.9.0+
• git diff --check

Fixes #6027

Greptile Summary

This PR bumps the documented and enforced Node.js minimum version from 18 to 20.9.0 on the release/3.12 branch, aligning README.md and package.json with the existing migration guide and the Node version already in use by CI workflows.

• README.md: Updates the "Requirements" bullet from "Node 18+" to "Node 20.9.0+".
• package.json: Updates engines.node from >=18 to >=20.9.0, so package managers that respect the engines field will now surface a warning/error for users on older Node versions.

Confidence Score: 5/5

Safe to merge — only the documented and enforced Node minimum version is updated; no logic changes.

Both changed files are consistent with each other and with the CI workflows, which already pin Node 20. The remaining references to "18" elsewhere in the repo are example placeholders in issue/PR templates, not version requirements.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Updates Node.js requirement from "Node 18+" to "Node 20.9.0+" in the Requirements section.
package.json	Updates engines.node from >=18 to >=20.9.0, aligning the enforced engine constraint with the documented requirement.

_{Reviews (1): Last reviewed commit: "docs: require Node 20.9 for release 3.12" | Re-trigger Greptile}

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