Fix CI (io_uring not supported)

[Thomasdezeeuw]

Currently the CI uses Linux 5.15, which doesn't support operations such as the send(2) system call, which is a bit of a pain. The next Linux LTS, v6.1, does support everything we need so maybe it's just a matter of waiting until Ubuntu LTS starts using that.

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I've split up and disable the CI for now (in https://github.com/Thomasdezeeuw/heph/pull/596), list to enable:

• [ ] Heph
• [ ] Heph-rt
• [x] Heph-inbox
• [ ] Heph-HTTP
• [ ] Heph-remote

Add support for testing under miri:

• [ ] Heph (after we remove the test dependency of Heph-rt
• [x] Heph-inbox.

For the other crates we can't as Heph-rt doesn't work under miri and the other crates depend on it.

[Thomasdezeeuw]

For context https://github.com/Thomasdezeeuw/heph/pull/577 introduced the usage of io_uring.

[Thomasdezeeuw]

I think we'll have to wait a couple more month until Ubuntu 24.04, that should work.

[Thomasdezeeuw]

I have since gotten the CI to work for A10, but I'm not sure if it's worth the effort Heph at the moment.