I wouldn’t ignore them or flat out say it. I think it’s best just to slowly hang out with them less and less. That way, they won’t be offended or hurt and will just think that you’re so busy meeting new friends that you forgot to include them.
Sometimes being kind is better than being honest, in this case. No one wants to think of themselves as “undesirable”. Conversely, they may be thinking the same about you, so it may all work out anyways, since people do change and maybe they have sensed the same change as well.
Another reason this works so well is, that it allows the possibility for you to be friends with these people in the future, should your interests be the same again. You just never know.