Bump supported Varnish versions

[TwoD]

I've been having issues with the currently supported versions taking a very long time to start (between 5-30minutes).
Running a strace on the process shows it closing countless bad file descriptors until the child process finally starts.

I've run into this on multiple (Arch based) machines both with Lando and when running the container directly.
After a quick discussion on the Varnish Discord we arrived at the conclusion that the LTS version (6.0.11) has likely not had a patch for this backported, and Lando does not support anything newer without a custom service.

Additionally, the Docker image dropped support for Varnish 4 as of yesterday, so it would be a good time to catch up: wodby/varnish@acb5489