The yml format is not supported by the YamlEncoder

**Symfony version(s) affected**: 4.1.6

**Description**  

The `YamlEncoder` currently support only the `yaml` format:

https://github.com/symfony/symfony/blob/cbe289517470eeea27162fd2d523eb29c95f775f/src/Symfony/Component/Serializer/Encoder/YamlEncoder.php#L25

AFAIK, the `.yml` file format is also accepted for Yaml format. IMO, it should be added to the support of this encoder.

