patch 9.0.0593: CI actions have too many permissions

Problem:    CI actions have too many permissions.
Solution:   Restrict permissions to what is required. (closes #11223)
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6c1b359..c988949 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,6 +12,9 @@
   group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
   cancel-in-progress: true
 
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
 jobs:
   linux:
     runs-on: ubuntu-20.04