Option to disable HTTP2 when behind external proxy

[MB-Finski]

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Description

An option to disable HTTP2 (h2,h2c) from GUI would be desirable and its use should be advised if nextcloudpi is placed behind a reverse proxy.

HTTP2 causes issues when nextcloudpi is behind a NGINX reverse proxy because nextcloudpi will return "Upgrade: h2,h2c" header for every request. This will cause mobile Safari clients to fail to connect to Nextcloudpi altogether. On the other hand, if the upgrade header is removed, notify_push websocket connection will no longer work since in this instance the "Upgrade: websocket" header will be removed.