Change nozzle size to variable reference

[the-ercules]

https://github.com/prusa3d/PrusaSlicer-settings/blame/280a1ad7546f42ec1b288f4ec0b66dd5a6fdde86/live/PrusaResearch/1.14.2.ini#L21477

While multiple nozzle sizes for MMU3 on MK4 are not yet supported, hard-coding nozzle size in the start G-Code is sub-optimal.

I've modified the second line of my own start G-Code from this:
M862.1 P0.4 ; nozzle diameter check
to this
M862.1 P{nozzle_diameter[0]} ; nozzle diameter check

Users still have to go in and change the extruder nozzle size, but this way users don't ALSO have to change the start G-Code. If they don't change the start G-Code, they'll receive a message saying the nozzle size is incompatible on the printer. This can be bypassed, but it gets the user into a bad habit of immediately disregarding these warnings.