Update seastar submodule
Update seastar submodule

* seastar ebcb3aeec...35c255dcd (1):
  > append_challenged_posix_file_impl: allow destructing file with no queued work
Fixes #7285.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla