Clean HTTP 1.1 to 2 upgrade headers

[NiccoMlt]

When proxying a request, we currently copy all headers without modification:

carapaceproxy/carapace-server/src/main/java/org/carapaceproxy/core/ProxyRequestsManager.java

Line 438 in 3b1d472

req.headers(request.getRequestHeaders().copy()); // client request headers

This approach causes problems when our server, configured with HTTP/1.1, receives a request from a client that supports HTTP/2. For example, consider the following cURL request:

curl --http2 -v http://<Carapace URL>

The HTTP/1.1 request contains the following headers:

Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <token68>

Our proxy forwards these headers to the backend server. If the backend server supports HTTP/2, it attempts to upgrade the connection. The proxy then forwards this response to the client, which tries to upgrade its connection with the proxy.

However, since the server part doesn't support HTTP/2 yet, the connection fails with the error "Invalid connection preface presented".

To resolve this issue, we should drop the following headers when copying headers during forwarding:

• Connection (MDN)
• Upgrade (MDN)
• HTTP2-Settings (no MDN page found)