Skean V0.2.1: bootloader fix

Adafruit's nRF52 bootloader support for the 52833 SoC was pretty new when this project was made, so the bootloader here was still using the 52840 identification.
Now, both the bootloader and ZMK (since its Zephyr 3.0 transition) support the 52833 properly. Unfortunately, that means boards with the old bootloader won't accept fresh firmware.
To fix that, just load up bootloader_update.uf2 like any old regular file! After that, the skean branch for zmk-config should provide a base for all your keymaps - it's already been updated for the latest transition.