gha: run php-cs-fixer with minimum supported PHP verrsion

Author: mfn

.github/workflows/autoformat.yml

@@ -52,7 +52,7 @@ jobs:
         with:
           coverage: none
           extensions: mbstring
-          php-version: 8.1
+          php-version: 7.2
 
       - run: composer install --no-interaction --no-progress --no-suggest