[php-symfony] Symfony6 support

Created by: BenjaminHae

Update php-symfony's generator template to work with Symfony6 and php8. fix https://github.com/OpenAPITools/openapi-generator/issues/11322

This PR is based on #11339 and #10763.

I've explicitly decided not to create a new generator php-symfony6 as the current php-symfony generator only works with symfony 4.x which is under LTS support. It is very unlikely that symfony 4.xis used for new projects. Symfony 5.x is currently unsupported by openapi-generator.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request. @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ackintosh (2017/09) ❤️, @ybelenko (2018/07), @renepardon (2018/12)