Consider that the cells don't know they're in a pack. Your responsibility is to each cell. Each cell requires 4.2V to charge effectively, so that's what you have to provide.
The simplest way to do that is to charge at up to 21V, and hope that the voltage is evenly distributed between the cells. Generally it will be, unless one of the cells is significantly weaker than the others, or damaged. If so, it may exhibit an internal resistance or cell voltage significantly different to the other cells, and therefore see a different voltage to the other cells.
Often these non-ideal effects are compensated for by individual cell electronics, but if you don't have that luxury then you can still provide some protection by simply setting an overall current limit. Without the current limit, you risk destroying cells or even starting a fire.
A sensible current limit for Lithium Ion cells is generally "0.5C - 1.0C", which means half the one-hour capacity to one times the one-hour capacity. Your cells are 2.15Ah, so 1A is a safe current limit, and you can go up to 2.15A if you need to charge quicker.