Update setup.rst

[carlcasbolt]

compose.json suggests 7.2.9 is required for version 5

[javiereguiluz]

@carlcasbolt thanks for this contribution! Sadly, this is trickier than it looks at first sight.

First, we showed 7.2.9 (see #12312) but a few days ago, we fixed it and lowered to 7.2.5 (see #12693).

There was a temporary Symfony issue that prevented using 7.2.5 and 7.2.9 was the minimum requirement in practice. But this was solved a couple of days ago, so it should be 7.2.5 again.

If you have problems trying to create a Symfony 5 app that requires 7.2.5, please share some details so we can fix the cause of that problem. Thanks!