before this change, we rely on the default-generated fmt::formatter created from operator<<, but fmt v10 dropped the default-generated formatter. in this change, we define formatters for `cached_promoted_index::promoted_index_block`, and drop its operator<<. Refs #13245 Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Closes scylladb/scylladb#17415