chore: update github workflow to php 8.1

Author: Laurent Bientz

.github/workflows/continuous-integration.yml

@@ -25,7 +25,7 @@ jobs:
  php-version:
  - "7.2"
  - "7.4"
- - "8.0"
+ - "8.1"
  dependencies:
  - "highest"
  include: