fix #4779: client: add missing "Connection" header for HTTP2 upgrade

This commit adds the missing "Connection: upgrade" HTTP header [1]
when requesting an upgrade to HTTP 2.
Doing so is mandated in the HTTP Semantics RFC [2], and without this,
(reverse) proxies that strictly follow the standard could potentially
break.

[1]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Upgrade
[2]: RFC 9110, 7.8. Upgrade: “[...] sender of Upgrade MUST also send
     an "Upgrade" connection option in the Connection header [...]”

Reported-By: McTwist <rajb89@hotmail.com>
Signed-off-by: Max Carrara <m.carrara@proxmox.com>
 [ TL: added RFC reference and use case to commit message ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

pbs-client/src/http_client.rs

@@ -764,6 +764,8 @@ impl HttpClient {
             );
         }
 
+        req.headers_mut()
+            .insert("Connection", HeaderValue::from_str("upgrade").unwrap());
         req.headers_mut()
             .insert("UPGRADE", HeaderValue::from_str(&protocol_name).unwrap());