Update Pico SDK to 2.1.1

[dhalbert]

Has a number of fixes and allows higher clock rates.
https://github.com/raspberrypi/pico-sdk/releases/tag/2.1.1

[eightycc]

Higher clock rates were already allowed by SDK 2.1.0. SDK 2.1.1 adds PICO_USE_FASTEST_SUPPORTED_CLOCK that when set selects 200MHz as the core clock frequency. Note that this is a build-time setting. It has no effect on the ability of CP to alter the core clock frequency at run time.