Improve composer consistency

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR addresses issue #2172.
Todo list:
- [x] unclutter sylius/sylius composer.json
- [x] check for version discrepancies
- [x] try to avoid using "minimal-stability": "dev"