Fix permissions of release-milestone workflow

Author: snicoll

.github/workflows/release-milestone.yml

@@ -4,6 +4,8 @@ on:
     tags:
       - v5.0.0-M[1-9]
       - v5.0.0-RC[1-9]
+permissions:
+  contents: read
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
 jobs: