Enable the Serializer

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | ~

With symfony/symfony#12295, the Serializer is fully functional and can be enabled by default.
Enabling it in the standard edition avoid BC.