Add missing return type (#30)

To fix deprecation:

```
Method "Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load()" might add "void" as a native return type declaration in the future. Do the same in implementation "Zeichen32\GitLabApiBundle\DependencyInjection\Zeichen32GitLabApiExtension" now to avoid errors or add an explicit @return annotation to suppress this message.
```

Allow symfony 7 (#29)

Add TreeBuilder return type. (#27)

Update dependency requirements - allowed Symfony 6 (#23)

Update dependency requirements - allowed Symfony 6

Update Configuration.php (#21)

Add m4tthumphrey/php-gitlab-api 11.x compatability (#20)

* Upgrade to m4tthumphrey/php-gitlab-api 11.x compatability

* Bump branch alias to 6.x

* Update readme.md to show the latest version requirement.

Allow PHP 8 (#18)

* Allow PHP 8
* Add PHP 8 to Travis builds
* Update deps to support PHP 8

Fix builds

Current changes (#14)

* Update Readme (PR #12)
* Fix branch alias (PR #13)
* Avoid deprected create method(PR #11)
* Update deps and install docs

Thanks to @GrahamCampbell for contribution.

Update composer requirements (#9)

- Use phpunit 8
- Update unit tests
- Only add supported symfony versions to composer.json
- Only add supported php version to travis.yaml