variant/mkrnb1500 SARA R410 UART not configured to utilize flow control (RTS/CTS)

the schematic of the MKRNB1500 includes support for flow control (RTS/CTS) between the samd21 and the ublox SARA r410 modem.

when looking at the implementation; the Serial UART fails to include the flow control lines, when the GSM 1400 does

```
variant/mkrnb1500/variant.cpp
// SerialSARA
Uart Serial2(&sercom4, PIN_SERIAL2_RX, PIN_SERIAL2_TX, PAD_SERIAL2_RX, PAD_SERIAL2_TX);

void SERCOM4_Handler()
{
Serial2.IrqHandler();
}

variant/mkrgsm1400/variant.cpp
// SerialGSM
Uart Serial2(&sercom4, PIN_SERIAL2_RX, PIN_SERIAL2_TX, PAD_SERIAL2_RX, PAD_SERIAL2_TX, PIN_SERIAL2_RTS, PIN_SERIAL2_CTS);

void SERCOM4_Handler()
{
Serial2.IrqHandler();
}
```

to enable flow-control; the SerialSARA (Serial2) definition simply needs to be updated:

```
variant/mkrnb1500/variant.cpp
// SerialSARA
- Uart Serial2(&sercom4, PIN_SERIAL2_RX, PIN_SERIAL2_TX, PAD_SERIAL2_RX, PAD_SERIAL2_TX);
+ Uart Serial2(&sercom4, PIN_SERIAL2_RX, PIN_SERIAL2_TX, PAD_SERIAL2_RX, PAD_SERIAL2_TX, PIN_SERIAL2_RTS, PIN_SERIAL2_CTS);

void SERCOM4_Handler()
{
Serial2.IrqHandler();
}
```

Open variant/mkrnb1500/variant.cpp and compare the SerialSARA Serial2 definition with the corresponding SerialGSM definition in variant/mkrgsm1400/variant.cpp. Confirm the MKRNB1500 UART uses its defined RTS and CTS pins, then build the SAMD core or run the repository's available checks to verify the change.

Written by the indexing model from the issue text.