fix(ci): Remove parallelisation of CI jobs to make it more robust

This is a temporary fix.

Refs: #163

Author: damienalexandre

.github/workflows/ci.yml

@@ -43,6 +43,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             fail-fast: false
+            max-parallel: 1 # Avoid rate-limit issue on Slack API
             matrix:
                 php-version: [ '8.0', '8.1', '8.2' ]
                 composer-flags: [ '' ]