Update default.conf

[yxie2023]

ipv6 support is automatically added via nginx docker's 10-listen-on-ipv6-by-default.sh.

This patch allows element-web to run on system with or without ipv6 enabled.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[CLAassistant]

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