Move to other nrf5x-firmware

[dchristl]

This is the continuing discussion on pull request #117

Current state:

• [heystack-nrf5x(https://github.com/pix/heystack-nrf5x) seeems a good alternative with more options
• Doesn't support battery flag completely (indicator if option is active not implemented ) > older devices /otehr firmwares show always full battery
• first tests with patched firmware (with implemented battery indicator) are promising

[Toastee0]

I was playing with this and have an unstested, (but it compiles and flashes to my xiao nrf54l15) firmware available here:
https://github.com/Toastee0/Seeed-Xiao-nRF54L15/tree/main/bluetooth/findmy