repair_service::insert_repair_meta gets the reference to a table
and passes it to continuations. If the table is dropped in the meantime,
the reference becomes invalid.
Use find_column_family at each table occurrence in insert_repair_meta
instead.
Fixes: #20057
(cherry picked from commit 719999b34c)
Refs #19953
Closes scylladb/scylladb#20077