Enhance Nginx reverse proxy configurations for HTTP/3 and SOGo

[aronmal]

This commit updates the documentation to provide crucial configuration adjustments for Nginx reverse proxies utilizing HTTP/3, specifically addressing SOGo redirect issues within Mailcow.

Two key nginx areas are covered:

1. reuseport Directive Management:
   The reuseport directive, while useful for opening UDP port on the same as the TCP port on a virtual server instance, should only be used once (like always with default_server) when used in HTTP/3 configurations. Disabling reuseport except for one occurrence has resolved issues where nginx reports errors ...: duplicate listen options for 0.0.0.0:443 in /etc/nginx/....

2. Correct Host Resolution for SOGo Redirects:
   After a successful login, SOGo issues 302 temporary redirects to the user's mailbox. With HTTP/3, the proxy_set_header Host $http_host; directive result in SOGo generating redirects with an incorrect or missing hostname (e.g., https:///SOGo/...). Switching to proxy_set_header Host $host; ensures the correct hostname is passed to SOGo, allowing it to construct accurate redirect URLs.

• SOGo redirects to local hostname if NGINX reverse Proxy has QUIC/HTTP3 enabeld mailcow-dockerized#5411
• Alias Problem mailcow-dockerized#2468

These clarifications will help users configure their Nginx reverse proxies to maintain seamless SOGo functionality when enabling HTTP/3.