chore(docs): Switch from master to next for pulling bbup's installation script (#16566)

## Problem

Master updates aren't necessarily cut frequently to reflect the latest
updates in bbup's installation script, causing frictions for its users.

#16541 for example,
the installation script on master was fetching bbup on master, which
lacked the latest list of version compatibility with the latest Noir
versions.

## Alternatives considered

We can leave the script as is, and when a new master merge happens it
will resolve itself.

I don't personally see benefits in maintaining the delay though, as the
script is mostly used by Noir + Barretenberg devs and has no role in
Aztec network's operations (hence no network instability introduced).

Author: signorecello

barretenberg/docs/docs/getting_started.md

@@ -18,7 +18,7 @@ Although it is a standandalone prover, Barretenberg is designed to be used with
 Inspired by `rustup`, `noirup` and similar tools, you can use the `bbup` installation script to quickly install and update Barretenberg's CLI tool:
 
 ```bash
-curl -L https://raw.githubusercontent.com/AztecProtocol/aztec-packages/refs/heads/master/barretenberg/bbup/install | bash
+curl -L https://raw.githubusercontent.com/AztecProtocol/aztec-packages/refs/heads/next/barretenberg/bbup/install | bash
 bbup
 ```
 

barretenberg/docs/versioned_docs/version-v0.87.0/getting_started.md

@@ -18,7 +18,7 @@ Although it is a standandalone prover, Barretenberg is designed to be used with
 Inspired by `rustup`, `noirup` and similar tools, you can use the `bbup` installation script to quickly install and update Barretenberg's CLI tool:
 
 ```bash
-curl -L https://raw.githubusercontent.com/AztecProtocol/aztec-packages/refs/heads/master/barretenberg/bbup/install | bash
+curl -L https://raw.githubusercontent.com/AztecProtocol/aztec-packages/refs/heads/next/barretenberg/bbup/install | bash
 bbup
 ```