Default compression silently changes when running under new Node version

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the regression has not already been reported

### Last working version

8.0.0

### Stopped working in version

8.1.0

### Node.js version

22.15+

### Operating system

Linux

### Operating system version (i.e. 20.04, 11.3, 10)

alpine 3.22

### 💥 Regression Report

When building an existing app using fastify-compress with a new version of node (going from 20 to 22 in our case), the default compression changes from `br` to `zstd` . It's quite unexpected that the default behavior should change based on which version of node the app is running on. This causes issues for us, since other parts of our stack do not expect (or indeed recognise) `zstd` compression.

### Steps to Reproduce

Run any project using `fastify-compress` under node < 22.15 . The default compression is `br`.
Run any project using `fastify-compress` under node > 22.15 . The default compression is `zstd`.

### Expected Behavior

Default compression should not change when node version changes