chore: add testrun for symfony 7

[Chris53897]

Please see #232 (comment)

Support for Symfony 7 is added in "phpspec/phpspec" 7.5.0
https://github.com/phpspec/phpspec/releases/tag/7.5.0

[dbu]

something is not working with those "LTS" builds - https://github.com/php-http/client-common/actions/runs/7600541313/job/20722422472?pr=237 is the one that says symfony 7, but it installs symfony 3 things 😱

we should probably set up flex with shivammatur like they do here doctrine/DoctrineBundle@42f5834 and make sure the major version is enforced.

[dbu]

the problem with the current setup: we install flex at the same time as updating the dependencies, so flex is not yet present and does not enforce versions.

[Chris53897]

@dbu
I think composer global was the missing part. But maybe you are right too.
Because i did remove more parts, because i think they are obsolete.
Please have a look if i did it correct.

I changed Symfony LTS according to the real LTS.
https://symfony.com/releases

[dbu]

ah, global is a solution as well.

and we missed to set the SYMFONY_REQUIRE variable? thanks for cleaning this up!