docs(migration): document Node.js 20 minimum version requirement

[nkaradzhov]

Description

Describe your pull request here

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

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Note

Low Risk
Documentation-only change to the migration guide; no runtime or API behavior is modified.

Overview
Adds a new Node.js minimum version section at the top of docs/v5-to-v6.md, before the existing RESP3 migration content.

The section states that v6 requires Node >= 20.0.0 (up from v5’s >= 18.19.0), ties the bump to @azure/msal-node 5.x via the entraid package (CVE-2026-41907 / transitive uuid), and notes that Node 18 is dropped from CI and npm install fails on Node 18.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}