bump mbed core to latest (#91)

Author: umbynos

.github/workflows/generate-index.yml

@@ -49,7 +49,7 @@ jobs:
         env:
           SAMD_V: 1.8.11
           MEGAAVR_V: 1.8.7
-          MBED_NANO_V: 2.2.0
+          MBED_NANO_V: 2.3.1
 
       - name: Install dependencies
         run: |