chore(ci): fix the Spectral rules URL

The rules where moved to another repository
(https://github.com/italia/api-oas-checker-rules), get them
from there.

.github/workflows/spectral.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - run: curl https://italia.github.io/api-oas-checker/spectral-full.yml > .spectral.yml
+      - run: curl https://github.com/italia/api-oas-checker-rules/releases/download/1.1/spectral-full.yml > .spectral.yml
 
       # Get additional module required by spectral-full
       - run: mkdir functions