Remove node-fetch alternative from HTTP client libraries recommendation

[Copilot]

This PR removes the outdated node-fetch alternative recommendation from the HTTP Client Libraries section in the template, as it's no longer needed for modern ioBroker adapter development.

Context

The template currently recommends node-fetch as an alternative for older Node.js versions:

### HTTP Client Libraries
- **Preferred:** Use native `fetch` API (available in Node.js 18+)
- **Alternative:** Use `node-fetch` for older Node.js versions
- **Avoid:** `axios` unless specific features are required (reduces bundle size)

However, ioBroker adapters now require Node.js 20+ as minimum (with Node.js 22 recommended), and the native fetch API has been available since Node.js 18+, making the node-fetch alternative obsolete.

Changes

• Removed the line recommending node-fetch for older Node.js versions
• Updated the preferred option description to clarify Node.js version requirements: "available in Node.js 18+, required Node.js 20+ for adapters"
• Maintained all existing code examples and other recommendations

Impact

This change provides clearer guidance to adapter developers by removing confusion about which HTTP client to use, aligning the template with current ioBroker ecosystem requirements.

Fixes #45.

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[Copilot]

Pull Request Overview

This PR updates the documentation to align with the minimum Node.js 20+ requirement by removing the outdated node-fetch alternative and clarifying the recommended HTTP client.

• Remove node-fetch as an alternative
• Clarify that adapters require Node.js 20+ while fetch is available since Node.js 18