Remove --legacy-peer-deps once starlight-llms-txt supports Astro 7

[tomarra]

Context

Dependabot bumped `astro` to v7 (#571), but `starlight-llms-txt@0.10.0` currently requires `astro@^6.0.0` as a peer dependency. This caused `npm install` to fail with `ERESOLVE`.

As a temporary workaround, `--legacy-peer-deps` was added to the install step in `.github/actions/astro_site/action.yaml`.

Action required

Once delucis/starlight-llms-txt#127 is merged and a new version is released with Astro 7 peer support:

1. Bump `starlight-llms-txt` to the new version in `package.json`
2. Remove `--legacy-peer-deps` from `.github/actions/astro_site/action.yaml`