BUG/MINOR: h3: filter upgrade connection header

BUG/MINOR: h3: filter upgrade connection header

As specified in RFC 9114, connection headers required special care in
HTTP/3. When a request is received with connection headers, the stream
is immediately closed. Conversely, when translating the response from
HTX, such headers are not encoded but silently ignored.

However, "upgrade" was not listed in connection headers. This commit
fixes this by adding a check on it both on request parsing and response
encoding.

This must be backported up to 2.6.