crimson: use the seastar span sink API and build at API level 10

crimson: use the seastar span sink API and build at API level 10

At SEASTAR_API_LEVEL >= 9 seastar dropped output_stream::write(net::packet) in
favour of write(std::span<temporary_buffer<char>>). Convert Socket's writes to
the span form, writing the packet's released fragments and keeping them alive
across the write with do_with, then raise Seastar_API_LEVEL from 6 to 10 to
match the updated seastar. The span write overloads are unconditional, so the
Socket change is valid at every API level.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>