Use 'composer audit' instead of 'symfony check:security'

[rosier]

Since Composer 2.4, there is a new command called audit, that outputs a list of reported security vulnerabilities for the list of packages versions currently installed.

This removes the need to download and install Symfony CLI

[rosier]

Add the moment there is a security vulnerability advisory. So the failure in https://github.com/symfony/demo/actions/runs/3183707333/jobs/5191246090 is expected

[stof]

@javiereguiluz I suggest meging this.
The current CI setup is broken anyway due to using the wrong path for the symfony CLI.

[javiereguiluz]

Thank you Jan.