docs: replace 'whitelist' with 'allow' for inclusive language

[tennisleng]

Description

This PR updates the README to replace the term 'whitelist' with 'allow' in the High Availability section, using more inclusive language.

Changes

• Replaced 'whitelist' with 'allow' in the firewall configuration note
• No functional changes, documentation only

Rationale

Many open source projects are moving away from terms like 'whitelist/blacklist' to more inclusive alternatives like 'allow/block' or 'allowlist/denylist'. This change aligns with modern documentation best practices.

Type of Change

• [x] Documentation update

Checklist

• [x] Documentation-only change
• [x] Follows project style guidelines
• [x] No functional changes