Update PyOCD test version

Author: thegecko

README.md

@@ -1,4 +1,4 @@
 # CMSIS Debug Adapter
-Debug adapter for CMSIS-DAP following the [DAP](https://microsoft.github.io/debug-adapter-protocol/) protocol. Tested with pyOCD version >= 0.14.0
+Debug adapter for CMSIS-DAP following the [DAP](https://microsoft.github.io/debug-adapter-protocol/) protocol. Tested with pyOCD version >= 0.21.0
 
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