Fix Rector dependency resolution

[dereuromark]

The cs-stan job started failing while Rector initialized:

MissingPrivatePropertyException: Property "$container" was not found in "PHPStan\Parser\RichParser"

Rector 2.3.9 allows PHPStan 2.2, but its container integration still depends on the PHPStan 2.1 RichParser implementation. Constrain the temporary Rector installation to PHPStan 2.1 until Rector can be upgraded.

This keeps the existing Rector 2.3 rule set. Upgrading Rector instead would make the dry run propose changes to 120 files.

Verified with:

composer rector-setup
composer rector-check
composer cs-fix
composer phpstan
vendor/bin/phpunit tests/TestCase/TestSuite/MigratorTest.php