Release 2.1.0

Core

RP2040: improve the overall user experience
RP2040: fix some relevant bugs (complete list here https://github.com/ARMmbed/mbed-os/pull/14488/commits)
USBSerial: speedup operations and tackle corner cases

Libraries

PDM: rationalize gain() and buffer size
Ethernet: fix library discovery on case insensitive platforms
Wire: fix I2C scan

Extra

Update openocd to its latest release (fixes a bug with nRF52)