Add SerialTCPClient library (#7281)

* Update repositories.txt

Add https://github.com/mobizt/ReadyMail repository

* Add SerialTCPClient repository

Author: mobizt

repositories.txt

@@ -4748,6 +4748,7 @@ https://github.com/mobizt/Firebase-ESP32
 https://github.com/mobizt/Firebase-ESP8266
 https://github.com/mobizt/FirebaseJson
 https://github.com/mobizt/ReadyMail
+https://github.com/mobizt/SerialTCPClient
 https://github.com/modulolabs/modulo-lib
 https://github.com/mogel77/Jobber
 https://github.com/MohammedRashad/ArduZ80