ci: test composer 2.9 (#596)

see https://github.com/composer/composer/releases/tag/2.9.0

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>

Author: jkowalleck

.github/workflows/php.yml

@@ -167,6 +167,7 @@ jobs:
           - "8.1"  # lowest supported
         composer:
           - "v2"   # latest 2.x
+          - "2.9"
           - "2.8"
           - "2.7"
           - "2.6"