Added return types to phpdoc comments to fix deprecations

[stollr]

This PR fixes the following deprecation messages by adding a "soft" return type to the phpdoc comments (to keep BC):

Method "Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load()" might add "void" as a native return type declaration in the future. Do the same in implementation "TheCadien\Bundle\SuluImportExportBundle\DependencyInjection\SuluImportExportExtension" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface::process()" might add "void" as a native return type declaration in the future. Do the same in implementation "TheCadien\Bundle\SuluImportExportBundle\DependencyInjection\Compiler\DbConnectionPass" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Config\Definition\ConfigurationInterface::getConfigTreeBuilder()" might add "TreeBuilder" as a native return type declaration in the future. Do the same in implementation "TheCadien\Bundle\SuluImportExportBundle\DependencyInjection\Configuration" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Console\Command\Command::configure()" might add "void" as a native return type declaration in the future. Do the same in child class "TheCadien\Bundle\SuluImportExportBundle\Command\ImportCommand" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Console\Command\Command::execute()" might add "int" as a native return type declaration in the future. Do the same in child class "TheCadien\Bundle\SuluImportExportBundle\Command\ImportCommand" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Console\Command\Command::configure()" might add "void" as a native return type declaration in the future. Do the same in child class "TheCadien\Bundle\SuluImportExportBundle\Command\ExportCommand" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Component\Console\Command\Command::execute()" might add "int" as a native return type declaration in the future. Do the same in child class "TheCadien\Bundle\SuluImportExportBundle\Command\ExportCommand" now to avoid errors or add an explicit @return annotation to suppress this message.

[TheCadien]

@stollr thanks for your effort for the bundle and the PRs ! I think it makes sense to give the bundle some real Github actions like a PHPStan check :D

[stollr]

@TheCadien Are you planning to add a release containing this change (or for #9)? I'd like to reduce the deprecation messages in my project 😅