feat!: migrate to sttp 4

[limansky]

sttp v4 is available for a while. It is stable, provides simpler API comparing to v3. I've adopted the code for the new sttp, however I have no real use case to test how does it work.

[ex0ns]

Thank you very much for your contribution, I quickly went through the PR and I believe that it makes sense to upgrade, it also clean up a few thing and more importantly it remove some implicit context (decoder/encoder + the async http client being created in the test example).
I make my best to review and test this thoroughly as soon as possible

This is a breaking change, so I will have to release 7.0, I will see if they are other breaking changes I should ship at the same time, maybe the Pekko one.

[ex0ns]

I tested quite a few examples and they work well, there is only one thing that broke inside the launcher for the ZIO bots.
Also with the fix that I just published for src-jvm-2 there are a few examples that need to be upgraded.

[ex0ns]

Working great, if you can address the ZIO Commands' bot issue, I will then merge this PR 👌