Update WebTestCase to be compatible with Symfony 7

[stof]

No description provided.

[stof]

hmm, this change relies on full variance support to be compatible with Symfony 6. I see 2 ways to solve that

• bump the min PHP version to PHP 7.4
• rework the custom WebTestCase class to override less methods from the parent.

[stof]

@monteiro any preference for the solution ?

[monteiro]

I think we can bump the version.

[stof]

@monteiro I updated the PR to bump the min PHP version.

[monteiro]

thanks!