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There really is no reason to formally end an old friendship. The earlier answer by @svj is exactly right in saying that

if you really want to end old friendships, pretend to be busy whenever they call and never call back (...) frequency of calls you get reduces and over a period of time, everyone kind of gets the point that you are not interested in keeping up the friendship (...) I don't think you should end a friendship unless the relationship is toxic or the particular friend is a negative person and you don't want them in your life.

It is as easy as ignoring them to give a clear non-verbal message that you do not want to continue the friendship. On the other hand, telling them outright that you want to end the friendship would be hurtful and I would advise against it for that reason. Even if the friend were toxic I would advise you to simply ignore them, because they don't deserve an explanation, though your friends don't appear toxic, judging by your question.

The real value of old friends, in my experience, is that they will remain much closer to you than any new friends, even if you are not in regular contact, and old friends can be extraordinarily supportive (especially emotionally) if you ever find yourself in a difficult situation, even if you have ignored them in the interim. You lose that extremely valuable source of support by trying to formally end the friendship.

Meanwhile, don't feel guilty for ignoring them. People can accept that you have other friends and other interests: it has happened to us all and we have all done it as well! They will certainly understand and the friendship can be kept on hold for the future.