Modernize bundle structure: AbstractBundle, PHP config, exception types, dependency constraints

[loevgaard]

Collecting the structural items in one issue. Most of it is already known (the XML loader deprecation is noted in the extension) and tied to Symfony 8 support.

1. AbstractBundle and PHP service config. XML DI configuration is deprecated in Symfony 7.4 and removed in 8.0. Moving to Symfony\Component\HttpKernel\Bundle\AbstractBundle with configure(DefinitionConfigurator $definition), loadExtension(array $config, ContainerConfigurator $container, ContainerBuilder $builder) and config/services.php removes the separate Extension/Configuration classes and the Resources/ directory. AbstractBundle expects the modern layout (config/ at the bundle root), so this pairs with moving src/Resources/config to config/.
2. Exception types. SetonoMetaConversionsApiExtension::load() uses Webmozart\Assert\Assert::true()/keyExists() (src/DependencyInjection/SetonoMetaConversionsApiExtension.php:39-44), which throw InvalidArgumentException. Symfony convention for "bundle X must be registered" is \LogicException, and configuration errors should be InvalidConfigurationException. The current integration test even asserts on \InvalidArgumentException. With AbstractBundle the check becomes $builder->hasExtension('setono_tag_bag') inside loadExtension().
3. Configuration::isTagBagBundleInstalled() (src/DependencyInjection/Configuration.php:38) makes the configuration schema depend on installed packages, so config:dump-reference differs between installations. Reasonable trade-off, but state it in the README next to the client_side.enabled default. The same InstalledVersions::satisfies() check is duplicated in the extension; keep one.
4. Composer constraints. symfony/event-dispatcher-contracts: ^2.5 || ^3.0: 2.x pairs with Symfony 5.4, which is no longer supported; ^3.0 is enough. symfony/messenger is required even for client-side-only installations; consider moving it to suggest with a compile-time check when server_side is enabled.
5. prepend() should go away entirely (prepend() replaces the application's default Messenger bus #12).