ci: Update permissions for GitHub attestations (#2481)

* Remove explicit content read permissions as not formally required.
   - c.f. https://github.com/actions/attest-build-provenance/tree/v1.1.2?tab=readme-ov-file#usage

Author: matthewfeickert

.github/workflows/publish-package.yml

@@ -32,7 +32,6 @@ jobs:
     permissions:
       id-token: write
       attestations: write
-      contents: read
 
     steps:
     - uses: actions/checkout@v4