Add Arduino_OPC_UA to Arduino library index. (#5320)

Author: aentinger

repositories.txt

@@ -894,6 +894,7 @@ https://github.com/arduino-libraries/Arduino_MultiWiFi
 https://github.com/arduino-libraries/Arduino_NiclaSenseEnv
 https://github.com/arduino-libraries/Arduino_NineAxesMotion
 https://github.com/arduino-libraries/Arduino_OAuth
+https://github.com/arduino-libraries/Arduino_OPC_UA
 https://github.com/arduino-libraries/Arduino_OplaUI
 https://github.com/arduino-libraries/Arduino_Opta_Blueprint
 https://github.com/arduino-libraries/Arduino_OV767X