Refactor how we define the available locales

[javiereguiluz]

I never liked the app_locales parameter: https://github.com/symfony/demo/blob/main/config/services.yaml#L9

But we need it for the route locale requirements.

I was digging into how to improve this ... and realized that if we use the standard enabled_locales config option from Symfony, the {_locale} parameter included in the URLs is automatically restricted to those values.

So, we can simplify code by moving to enabled_locales without losing any functionality.

[94noni]

best part is no part 👍🏻