[php-slim4] Decouple Slim4 codegen from Slim3

[ybelenko]

It was a mistake to extend Slim4 from Slim3 codegen. When I need to fix something in Slim4 I have to overwrite code from Slim3 somehow and it's not always possible. It's even more important that Slim3 codegen shouldn't be touched as soon as it's deprecated long time ago.

Refresh samples didn't change which is main goal of this PR.

Guys, please merge this PR asap, I need to rebase on this all other php-slim4 PRs.

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, 5.1.x, 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.

cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @renepardon

[ybelenko]

@wing328 Could you please look at that when you have time? Since you in the PHP context right now.

[ybelenko]

Since @wing328 is really busy and that PR not actually related to PHP, but Java code in general I would subscribe @OpenAPITools/generator-core-team

[wing328]

LGTM