I disagree with @FumbleFingers' answer: specifically the part of it that says It probably means they were boisterously and noisily enthusiastic about the whole process of "team selection", rather than that they were specifically keen on having Brown in Gryffindor.
The definitions you gave in your question are not wrong, but there is a specific usage of catcall to indicate (usually male) approval of an attractive member of the (usually) opposite sex. I put the 'usuallys' in because it could in theory be used for women catcalling at men, gay or bi men catcalling to other men, etc. - however it is almost exclusively used for straight men calling at women.
1 a shrill whistle or shout of disapproval made at a public meeting or performance: he walked out to jeers and catcalls
1.1 a loud whistle or a comment of a sexual nature made by a man to a passing woman: women were the objects of catcalls when they walked by the men’s barracks
I think catcalling is used here to indicate the boys' approval of the Lavender joining the team. In other words, the whole table "explodes with cheers" and the Weasley brothers specifically make some kind of call that indicates either that they are happy for girls (any girls) to be joining Griffindor, or that they particularly approved of this girl (because she was good-looking).
As @Persian Cat pointed out, catcall is very similar to wolf whistle: it's used in the same way, the only difference is that it's some kind of shout rather than an actual whistle.
Depending on how the call is made, it can translate to anything from "I'd like to give her one!" (explicitly sexual) to "You're pretty!" (fairly harmless approval). It can therefore be seen as derogatory or demeaning in some contexts but I would interpret it here as more like "hooray, the pretty girl is in Griffindor!" which Lavender probably took positively.