Release 2.3.1

Core

Rebuild variants on mbed 6.12

analogWrite() for Portenta doesn't crash anymore on HRTIM pins (eg. pin 3).

• The default frequency of the PWM on such pins is 720Hz

Serial: properly protect ringbuffer access

Libraries

PDM: fix gain on RP2040
WiFi: refactor to match new mbed-os APIs

Extra

Restore functionality as mbed library (sample project in the comment here )