docs: clarify Node.js version support in LTS table

Separate bundled Node.js versions from full version support range.
- Add column showing which Node.js versions bundle each Undici version
- Add column showing minimum required and tested Node.js versions
- Makes it clear that v5.x supports Node.js ≥14.0 despite being bundled in v18.x

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

README.md

@@ -622,11 +622,11 @@ and `undici.Agent`) which will enable the family autoselection algorithm when es
 
 Undici aligns with the Node.js LTS schedule. The following table shows the supported versions:
 
-| Version | Node.js     | End of Life |
-|---------|-------------|-------------|
-| 5.x     | v18.x       | 2024-04-30  |
-| 6.x     | v20.x v22.x | 2026-04-30  |
-| 7.x     | v24.x       | 2027-04-30  |
+| Undici Version | Bundled in Node.js | Node.js Versions Supported | End of Life |
+|----------------|-------------------|----------------------------|-------------|
+| v5.x           | v18.x              | ≥14.0 (tested: 14, 16, 18) | 2024-04-30  |
+| v6.x           | v20.x, v22.x       | ≥18.17 (tested: 18, 20, 21, 22) | 2026-04-30  |
+| v7.x           | v24.x              | ≥20.18.1 (tested: 20, 22, 24) | 2027-04-30  |
 
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