[RFC] Bump btf-rs to 1.2

[atenart]

RFC as btf-rs 1.2 is not released yet. This make Retis use the btf-rs BtfCollection.

[vlrpl]

I quickly went through this.
As a general comment, I'm in favor of this as it will be logically useful to decouple things in the outstanding effort in #496.
But other than that, things look more natural this way.
After a quick glance, I didn't spot anything worthy of comments code-wise, but I'll reserve the right to take a second look :) if/when we decide to move this forward.

Edit:

I didn't spot anything worthy of comments code-wise

of course, other than the infamous version bump in Cargo.lock, but we all know it is trying to sneak into the code and break c8s :)