docs: clarify Node.js version support in LTS table (nodejs#4375)

* 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>

* Update README.md

Co-authored-by: Aras Abbasi <aras.abbasi@googlemail.com>

---------

Signed-off-by: Matteo Collina <hello@matteocollina.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Aras Abbasi <aras.abbasi@googlemail.com>

Author: 3 people

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 |
+|----------------|-------------------|----------------------------|-------------|
+| 5.x           | 18.x              | ≥14.0 (tested: 14, 16, 18) | 2024-04-30  |
+| 6.x           | 20.x, 22.x       | ≥18.17 (tested: 18, 20, 21, 22) | 2026-04-30  |
+| 7.x           | 24.x              | ≥20.18.1 (tested: 20, 22, 24) | 2027-04-30  |
 
 ## License