Target Linux kernel 6.1

[armanbilge]

Linux 6.1 is the next LTS kernel, after 5.15 (which we are currently targeting).

So far, there is no Ubuntu LTS release based on this kernel and we cannot use it easily in GitHub Actions CI. So this PR sets up Cirrus CI with Ubuntu 23.04 and removes Circle CI. We still keep GHA jobs for building artifacts and publishing them, but we do not use it for testing.

Admittedly, this is a non-trivial jump to a kernel version that is not yet very widely deployed. Here's my thinking.

1. Somehow on 5.15 our use of io_uring appears to spawn several worker threads, which according to the developers is not the intended behavior ...
   Reading/writing on sockets starts several iou-wrk "workers" #56
   In any case, on 6.1 it is working as expected.

2. There are many useful and important features released after kernel 5.15 that we would do well to take advantage of.
   https://github.com/axboe/liburing/wiki/io_uring-and-networking-in-2023

3. Amazon Linux 2023 (the successor to Amazon Linux 2) ships with kernel 6.1.
   https://aws.amazon.com/linux/amazon-linux-2023/

So by upgrading we skip the performance-killing bugs, gain all the awesome new features, and still support the biggest cloud provider. Meanwhile, all this io_uring stuff is still pretty new and experimental anyway, so as it matures 6.1 should become more generally available.

Closes #64.

[armanbilge]

NB Ubuntu 23.04 is actually based on Linux kernel 6.2 🤔 I wonder if we can use AL2023 for CI somehow.

[armanbilge]

Doesn't look like there was really any new io_uring features in 6.2, just internal improvements. So, not ideal, but yolo 😅