Is rekeying attack the same as related-key attack? I've been searching about rekeying attack, but what I found is always using the term "rekeying/related-key attack". Is rekeying attack the same as related-key attack?
No, rekeying is about changing keys: https://en.m.wikipedia.org/wiki/Rekeying_(cryptography)
And we can have attacks ob the process of changing the key without need to have related keys. Which are two(or more) keys with some relation to each other which may lead to an attack.