v1.2.1 for Teensy41 and QNEthernet

### Releases v1.2.1

1. Initial porting and coding for **Teensy 4.1 using built-in QNEthernet**
2. Bump up version to v1.2.1 to sync with [AsyncUDP_STM32](https://github.com/khoih-prog/AsyncUDP_STM32) v1.2.1

Author: khoih-prog

changelog.md

@@ -12,7 +12,7 @@
 
 * [Table of contents](#table-of-contents)
 * [Changelog](#changelog)
-  * [Releases v1.2.1](#releases-v171)
+  * [Releases v1.2.1](#releases-v121)
 
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