examples: update max GPIO range for C2 examples to exclude UART0 pins

Closes https://github.com/espressif/esp-idf/issues/10174

examples/common_components/env_caps/esp32c2/Kconfig.env_caps

@@ -4,7 +4,8 @@ config ENV_GPIO_RANGE_MIN
 
 config ENV_GPIO_RANGE_MAX
     int
-    default 20
+    default 18
+    # GPIOs 19/20 are always used by UART in examples
 
 config ENV_GPIO_IN_RANGE_MAX
     int