ci: add alls-green action and consolidate workflows

- Merge pull-request-checks.yml and main-checks.yml into main.yml
- Add 'all-green' job using alls-green to aggregate all CI results
- Enables single required status check: 'all-green'

Author: Kludex

.github/workflows/main-checks.yml .github/workflows/main.yml.github/workflows/main-checks.yml renamed to .github/workflows/main.yml

@@ -1,4 +1,4 @@
-name: Main branch checks
+name: CI
 
 on:
   push:
@@ -8,6 +8,10 @@ on:
       - "v1.x"
     tags:
       - "v*.*.*"
+  pull_request:
+
+permissions:
+  contents: read
 
 jobs:
   checks: