chore: add testrun for "psr/http-message" v2

[Chris53897]

see https://github.com/php-http/message/issues/158

Just for testing the CI

[Chris53897]

I do not know how to fix the changes between slim 3 and 4. Did not find an easy migration path.

[dbu]

i am a bit afraid of testing with a dev version.

could we just remove slim from require-dev and add it with composer commands in the github ci for some builds but not all? and have the tests that depend on slim skip if slim is not available?

[Chris53897]

Yes of course. Maybe they will release a new version? https://github.com/slimphp/Slim/issues/3311

But i would suggest that the breaking-changes should be fixed first. At best from someone with more knowledge at this topic.

[dbu]

slim has tagged a release. can you please rebase this branch onto 1.x and push again? then we can see what problems remain.

[Chris53897]

@dbu Thanks Now i can work on the support for slim 4.