Update concepts-protocols.md
Update concepts-protocols.md

There is support for HTTP/2 trailers since 5.2
mitmproxy/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,Kriechi/mitmproxy,Kriechi/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy