MINOR: quic: Increase the QUIC connections RX buffer size (upto 64Kb)

With ~1500 bytes QUIC datagrams, we can handle less than 200 datagrams
which is less than the default maxpollevents value. This should reduce
the chances of fulfilling the connections RX buffers as reported by
Tristan in GH #1737.

Must be backported to 2.6.
diff --git a/include/haproxy/xprt_quic-t.h b/include/haproxy/xprt_quic-t.h
index b51779a..87127c2 100644
--- a/include/haproxy/xprt_quic-t.h
+++ b/include/haproxy/xprt_quic-t.h
@@ -258,7 +258,7 @@
 /* Size of the internal buffer of QUIC TX ring buffers (must be a power of 2) */
 #define QUIC_TX_RING_BUFSZ  (1UL << 12)
 /* Size of the QUIC RX buffer for the connections */
-#define QUIC_CONN_RX_BUFSZ (1UL << 14)
+#define QUIC_CONN_RX_BUFSZ (1UL << 16)
 
 extern struct trace_source trace_quic;
 extern struct pool_head *pool_head_quic_tx_ring;