update php-cs-fixer ruleset for php8

[jrushlow]

• adds the PHP 8 && PHPUnit 8.4 migration rulesets

• forces trailing commas in multiline parameter lists to improve diff readability. e.g.

public function __construct(
    private int $number,
    private string $word,
)

I also added trailing line commas in multiline argument lists and then reverted the commit. See commit 352f951

If this we should keep multiline trailing commas for args - I'll drop the commit that reverts it. Thoughts @weaverryan @OskarStark @wouterj ?

[weaverryan]

Thanks Jesse!