50-nixos-isoimage: Reference plasma6 ISOs for 24.05

[mweinelt]

The ISO name will change slightly due to the update from KDE Plasma version 5 to 6.

Blocked until we switch to 24.05 downloads on the homepage.

[thilobillerbeck]

Stupid question, but why don't we name the image just plasma? would fit more into the naming scheme imo.

[mweinelt]

That is a question for tada @K900.

[K900]

Because we may want to have both, at least for a while.

[mweinelt]

Superseded by #1453