raft: use `.contains` instead of `.count` for std::set in `raft::configuration::diff`
raft: use `.contains` instead of `.count` for std::set in `raft::configuration::diff`

`std::unordered_set::contains` is introduced in C++20 and provides
clearer semantics to check existence of a given element in a set.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

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