[nrf fromtree] Bluetooth: Host: Use MTU min for l2cap test

The many_conn l2cap test used a MTU lower than the minimum permitted
value. This commit bumps it to the minimum (23).

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>
(cherry picked from commit b799d18)
Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>

Author: HaavardRei

tests/bsim/bluetooth/host/l2cap/many_conns/src/main.c

@@ -29,7 +29,7 @@ DEFINE_FLAG_STATIC(flag_l2cap_connected);
 #define NUM_PERIPHERALS CONFIG_BT_MAX_CONN
 #define L2CAP_CHANS     NUM_PERIPHERALS
 #define SDU_NUM         1
-#define SDU_LEN         10
+#define SDU_LEN         23
 
 /* Only one SDU per link will be transmitted */
 NET_BUF_POOL_DEFINE(sdu_tx_pool,