[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

configuration.rst

@@ -839,20 +839,8 @@ In production, the ``.env`` files are also parsed and loaded on each request. So
 the easiest way to define env vars is by creating a ``.env.local`` file on your
 production server(s) with your production values.
 
-To improve performance, you can optionally run the ``dotenv:dump`` command (available
-in :ref:`Symfony Flex <symfony-flex>` 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):
+To improve performance, you can optionally run the ``dotenv:dump`` command.
+The command is not registered by default, so you must register first in your services:
 
 .. code-block:: yaml