feat(Aztec.nr): Remove the open keyword, in favour of the public attribute

[Maddiaa0]

Right now all open functions have a return value keyword at the beginning. This is then included in the abi and included in the contract's function tree. And also determines that brillig should be generated for the function. The use of the open keyword was chosen as public is considered overloaded. Consensus has such changed; the public keyword is now deemed suitable for this.

#[aztec(public)]
open fn some_function() {
    // ...
}

Should become:

#[aztec(public)]
fn some_function() {
    // ...
}

But maintain the same functionality

Tracked by #1891