BlueMicro2.0c RX/TX and Serial GPIOs potentially collide for some keyboards: Disable Serial by default when GPIO 6 and 8 are used by the matrix

[jpconstantineau]

Describe the bug
1 column doesn't work on my keyboard: GPIO for that Column collides with TX.

Additional context
Applies for BlueMicro V2.0c where RX and TX are mapped to Pro Micro header as well as the central programming header.