this is a part of a series to migrating from `operator<<(ostream&, ..)` based formatting to fmtlib based formatting. the goal here is to enable fmtlib to print `cdc::generation_id` and `db_clock::time_point` without the help of `operator<<`.
the formatter of `cdc::generation_id` uses that of `db_clock::time_point` , so these two commits are posted together in a single pull request.
the corresponding `operator<<()` is removed in this change, as all its callers are now using fmtlib for formatting now.
Refs #13245Closes#13703
* github.com:scylladb/scylladb:
db_clock: specialize fmt::formatter<db_clock::time_point>
cdc: generation: specialize fmt::formatter<generation_id>
72c12a1ab2