[configuration]: remove php<8 section, remove flex reference

- symfony 6.x requires php > 8 so configuration section for pre 8 is irrelevant
- `dotenv:dump` has been implemented in 5.4 and reference to flex is no longer relevant symfony/symfony@234c0b3

Author: kaznovac

configuration.rst

@@ -927,17 +927,6 @@ To improve performance, you can optionally run the ``dump-env`` Composer command
     1.2 or later). The command is not registered by default, so you must register
     first in your services:
 
-    .. code-block:: yaml
-
-        # config/services.yaml
-        services:
-            Symfony\Component\Dotenv\Command\DotenvDumpCommand:
-                - '%kernel.project_dir%/.env'
-                - '%kernel.environment%'
-
-    In PHP >= 8, you can remove the two arguments when autoconfiguration is enabled
-    (which is the default):
-
     .. code-block:: yaml
 
         # config/services.yaml