ci: Use least privilege principle for permissions

Declares only the required permissions per workflow.

Author: flovogt

.github/workflows/github-ci.yml

@@ -8,6 +8,9 @@ on:
     branches:
       - main
 
+# No permissions are required for this workflow
+permissions: {}
+
 jobs:
   test:
     name: General checks, tests and coverage reporting

.github/workflows/reuse-compliance.yml

@@ -8,6 +8,9 @@ on:
     branches:
       - main
 
+# No permissions are required for this workflow
+permissions: {}
+
 jobs:
   compliance-check:
     name: Compliance Check