io: make max unbounded io worker configurable

Since Linux 5.15 io_uring can limit the number of iow threads created
using IORING_REGISTER_IOWQ_MAX_WORKERS.

Bump liburing wrap to version 2.1 to use
io_uring_register_iowq_max_workers.

Expose this via the meson variable io_unbounded_iow_max and the
environment variable EMPER_IO_UNBOUNDED_IOW_MAX.

See for an detailed explanation:
https://blog.cloudflare.com/missing-manuals-io_uring-worker-pool