Update RTC_Alarm.ino

[karlsoderby]

Update RTC_Alarm to work with UNO R4 Minima & WiFi boards.

Changes made.

• Switch the LED used (from RGB > LED_BUILTIN). R4 boards have no RGB so this example does not compile/work.
• Made ledState global so that it can be used in loop() as well.
• Added serial functions to the loop to print the current state of the alarm. Alarm state is now printed each second.
• Moved alarmCallback() function to the bottom for better readability.

Example is tested and verified on both UNO R4 boards and the Portenta C33.

[aentinger]

I can confirm it compiles for Uno R4 WiFi, Minima and Portenta C33. LGTM 👍