Use pull_request triggers primarily for workflows

Instead of relying on push triggers in general, we now use pull_request
and reserve push triggers for main and release branches.

This makes the behaviour more consistent for users contributing from
repository forks. However, we no longer have a quick-feedback loop
for development.

.github/workflows/ci-check-and-unit-test.yml

@@ -2,6 +2,10 @@ name: CI-check-and-unit-test
 
 on:
   push:
+    branches:
+    - 'main'
+    - 'release/**'
+  pull_request:
 
 permissions:
   contents: read