Enable TIM8 update interrupt

Previously, heartbeat frames weren't actually sent because the timer
interrupt never fired.

Core/Inc/stm32g4xx_it.h

@@ -60,6 +60,7 @@ void FDCAN1_IT0_IRQHandler(void);
 void FDCAN1_IT1_IRQHandler(void);
 void TIM1_UP_TIM16_IRQHandler(void);
 void SPI1_IRQHandler(void);
+void TIM8_UP_IRQHandler(void);
 /* USER CODE BEGIN EFP */
 
 /* USER CODE END EFP */