ghcup: update metadata version to 0.0.9

[Erchiusx]

ADVISE WANTED

The ghcup team seems to be releasing patches for versions very frequently. As a result, the documentation often requires updates to reflect these changes. However, these updates and commits often feel redundant and contribute to polluting the commit history.

There seem to be two potential solutions to address this issue:

1. Provide a latest version for each metadata channel:
   This would involve creating the following files that always point to the latest version of the respective metadata:

   • ghcup-latest.yaml
   • ghcup-cross-latest.yaml
   • ghcup-vanilla-latest.yaml
   • ghcup-prereleases-latest.yaml

2. Make the site more dynamic:
   One option is to embed a JavaScript snippet that dynamically generates the latest version in the documentation. Alternatively, the site could be built daily, retrieving the latest version during the build process. While this could be generalized for other documents, it might be harder to maintain in the long term.

Additionally, these solutions are within our control. Beyond this, we could also consider reaching out to upstream to request that they provide the latest metadata directly to us, as a mirror site, so that we don't need to manage this manually.

[taoky]

1. **Provide a `latest` version for each metadata channel**:
   This would involve creating the following files that always point to the latest version of the respective metadata:
   
   * `ghcup-latest.yaml`
   * `ghcup-cross-latest.yaml`
   * `ghcup-vanilla-latest.yaml`
   * `ghcup-prereleases-latest.yaml`

I would prefer this: create a symlink to the latest one in the ghcup sync script.