osd/scrubber: replace deprecated fmt::localtime() with localtime_r()

osd/scrubber: replace deprecated fmt::localtime() with localtime_r()

Replace fmt::localtime() with localtime_r() to fix build failure with
fmt 11.2.0. The fmt::localtime() function was deprecated in favor of
std::localtime, causing build errors when treating warnings as errors:

```
[1/3] Building CXX object src/osd/CMakeFiles/osd.dir/scrubber/pg_scrubber.cc.o
/home/kefu/dev/ceph/src/osd/scrubber/pg_scrubber.cc: In member function ‘virtual void PgScrubber::update_scrub_stats(ceph::coarse_real_clock::time_point)’:
/home/kefu/dev/ceph/src/osd/scrubber/pg_scrubber.cc:2767:41: warning: ‘tm fmt::v11::localtime(time_t)’ is deprecated [-Wdeprecated-declarations]
 2767 |     auto printable_last = fmt::localtime(clock::to_time_t(m_last_stat_upd));
      |                           ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/kefu/dev/ceph/src/fmt/include/fmt/ostream.h:23,
                 from /home/kefu/dev/ceph/src/msg/msg_types.h:26,
                 from /home/kefu/dev/ceph/src/common/options.h:12,
                 from /home/kefu/dev/ceph/src/common/config.h:25,
                 from /home/kefu/dev/ceph/src/common/config_proxy.h:7,
                 from /home/kefu/dev/ceph/src/osd/scrubber/./pg_scrubber.h:78,
                 from /home/kefu/dev/ceph/src/osd/scrubber/pg_scrubber.cc:4:
/home/kefu/dev/ceph/src/fmt/include/fmt/chrono.h:538:28: note: declared here
  538 | FMT_DEPRECATED inline auto localtime(std::time_t time) -> std::tm {
      |                            ^~~~~~~~~
```

Unlike other parts of the codebase, this implementation checks the return
value of localtime_r() to preserve the error-handling behavior of
fmt::localtime(), which throws on failure. Future changes may opt for
consistency with the broader codebase over behavioral compatibility.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>