ci: Fix the SYMFONY_REQUIRE value

[theofidry]

Applying the suggestion from symfony/flex#1073 (comment)

[theofidry]

@stof @xabbuh FYI it does not appear that this version require fixed the problem:

Again: it's legit it didn't install symfony/framework-bundle v8 neither symfony/yaml v8 because I missed to adapt the constraint from "^6.0 || ^7.4" to "^7.4 || ^8.0" (I even missed bumping to 6.4 even apparently...). But I kinda expect the plugin to cause a failure in this case as we marked v8 as required, and it couldn't.

Adjusting the version like suggested did not change that.