Remove R23-12 from versioned kits

[arnoweiss]

Current Behavior

There's a decision that only two legacy versions should be supported. Currently, it's three (R23-12, R24-03, R24-05). Build times are quite long already.

Expected Behavior

Two versions back, quicker build.

Considerations

Even though R24-05 is out of the door, there may be some necessity to keep the docs for longer.