[VoxMesh]

The LilyGO LoRa32 V2.1-1.6 uses GPIO18 for LoRa NSS. GPIO18 is not RTC-capable on the original ESP32, so the call to rtc_gpio_hold_en() in ESP32Board::enterDeepSleep() logs RTCIO number error during shutdown.

Check rtc_gpio_is_valid_gpio() before that call. Handling of valid RTC GPIO and the ESP32-C3/C6 path is unchanged.

Tested:

• Firmware builds passed for ESP32, ESP32-S3, ESP32-C3 and ESP32-C6.
• pio test -e native -e native_kiss_modem passed.
• A local host check with mocked hardware APIs passes with this change and detects the invalid GPIO18 call in the unpatched function.

I haven't tested this on hardware.

Refs #3586