Switch to PSR-12

[ankitpokhrel]

PSR-2 is deprecated and PSR-12 is now recommended as an alternative. We need to update our cs-fixer config and apply changes to our codebase after PHP-CS-Fixer implements the @PSR-12 group.

Waiting for: PHP-CS-Fixer/PHP-CS-Fixer#4502

Provisional Ruleset

[
    '@PSR2' => true,
    'blank_line_after_opening_tag' => true,
    'braces' => ['allow_single_line_closure' => true],
    'compact_nullable_typehint' => true,
    'concat_space' => ['spacing' => 'one'],
    'declare_equal_normalize' => ['space' => 'none'],
    'function_typehint_space' => true,
    'new_with_braces' => true,
    'method_argument_space' => ['on_multiline' => 'ensure_fully_multiline'],
    'no_empty_statement' => true,
    'no_leading_import_slash' => true,
    'no_leading_namespace_whitespace' => true,
    'no_whitespace_in_blank_line' => true,
    'return_type_declaration' => ['space_before' => 'none'],
    'single_trait_insert_per_statement' => true,
];