Release 3.2.1 - FreeRTOS and I2S bug fixes, misc updates

Several important FreeRTOS bug fixes and update to latest upstream, minor I2S fix, and Adafruit Feather breaking change

Breaking changes

• BREAKING: Swap Wire1 and Wire in Adafruit Feather (#1461)

FreeRTOS bug fixes and updates

• Merge latest FreeRTOS/SMP upstream branch (#1449)
• Clean up FreeRTOS, remove core freeze hacks (#1448)
• Use real FreeRTOS tasks to idle core during flash (#1444)
• Fix FreeRTOS CoreMutex shim to handle ISRs (#1442)

Minor bug fixes and updates

• Fix I2S::write(const uint8_t *buffer, size_t size) (#1461)
• BT Serial connection docs for Mac (#1459)
• Add setInverted() options to SerialPIO (#1451)
• Add dummy Serial implementation when NO_USB defined (#1438)
• Generate warnings if Pico STDIO init called (#1467)