[Routing] correct environment-specific routing configuration examples

[santysisi]

Description

This PR updates the Symfony routing documentation examples to reflect the correct way to define environment-specific routes using when@dev (YAML), <when env="dev"> (XML), and conditional logic in PHP.

Changes Made

• YAML: Replaced env: dev with when@dev syntax block.
• XML: Wrapped the <route> element in a <when env="dev"> element.
• PHP: Added conditional logic using $routes->env() to only register the route in the dev environment.

Why?

The previous examples used either unsupported keys or incorrect syntax for defining environment-specific routes (e.g. using env: dev directly inside a route definition). This update replaces those with the correct environment-based separation using when@dev blocks (YAML), wrappers (XML), and runtime conditionals in PHP. It ensures that environment-specific routes are properly scoped and configured according to Symfony's routing system.

References

• Allowed keys in YAML routing — the env key is not permitted inside route definitions.
• RoutingConfigurator::env() method — this method returns the current environment and does not accept an environment name as an argument for filtering.
• RouteConfigurator class — does not have an env() method.

[javiereguiluz]

I wasn't aware that this routing config was so wrong! Santiago, thanks a lot for fixing it.