Draft: php-cs-fixer: enable trailing comma in multiline array again

this already happened for our frontend code in !1728 (merged) and with f77133dd

only the minimal, non-trivial changes are included in this commit to minimize potential merge conflicts.

a follow-up will contain the remaining, automated adjustments once we're ready to merge. you can check these in the "currently-"failing" backend build output.

we might also consider including the PhpCsFixer ruleset or parts of it as a next step, which has a few additional checks that I like. eg:

• array_indentation
• compact_nullable_typehint
• explicit_indirect_variable e.g. in deploy.php and run.php
• fully_qualified_strict_types
• phpdoc_add_missing_param_annotation with only_untyped

but also a few that I don't think will help our current codebase (we could disable them).

check which is which for youself: https://mlocati.github.io/php-cs-fixer-configurator