feat(cdc_acm): Add CDC-ACM support and remove old test applications

- Implemented CDC-ACM (USB Virtual COM Port) support for the slave_node application.
- Removed the now obsolete 'hello_world' and 'stm32g431_tests' applications.

software/apps/slave_node/cdc-acm.overlay

@@ -0,0 +1,10 @@
+&zephyr_udc0 {
+	cdc_acm_uart0: cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+
+		modbus0 {
+			compatible = "zephyr,modbus-serial";
+			status = "okay";
+		};
+	};
+};