replication_strategy: mark fmt::formatter<..>::format() const

since fmt 11, it is required that the format() to be const, otherwise
its caller in fmt library would not be able to call it. and compile
would fail like:

```
/home/kefu/.local/bin/clang++ -DFMT_SHARED -DSCYLLA_BUILD_MODE=release -DSEASTAR_API_LEVEL=7 -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SSTRING -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"RelWithDebInfo\" -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/seastar/gen/include -I/home/kefu/dev/scylladb/build/seastar/gen/src -I/home/kefu/dev/scylladb/build/gen -isystem /home/kefu/dev/scylladb/abseil -ffunction-sections -fdata-sections -O3 -g -gz -std=gnu++23 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/home/kefu/dev/scylladb=. -march=westmere -Xclang -fexperimental-assignment-tracking=disabled -mllvm -inline-threshold=2500 -fno-slp-vectorize -U_FORTIFY_SOURCE -Werror=unused-result -MD -MT locator/CMakeFiles/scylla_locator.dir/RelWithDebInfo/abstract_replication_strategy.cc.o -MF locator/CMakeFiles/scylla_locator.dir/RelWithDebInfo/abstract_replication_strategy.cc.o.d -o locator/CMakeFiles/scylla_locator.dir/RelWithDebInfo/abstract_replication_strategy.cc.o -c /home/kefu/dev/scylladb/locator/abstract_replication_strategy.cc
In file included from /home/kefu/dev/scylladb/locator/abstract_replication_strategy.cc:9:
In file included from /home/kefu/dev/scylladb/locator/abstract_replication_strategy.hh:16:
In file included from /home/kefu/dev/scylladb/gms/inet_address.hh:11:
In file included from /usr/include/fmt/ostream.h:23:
In file included from /usr/include/fmt/chrono.h:23:
In file included from /usr/include/fmt/format.h:41:
/usr/include/fmt/base.h:1393:23: error: no matching member function for call to 'format'
 1393 |     ctx.advance_to(cf.format(*static_cast<qualified_type*>(arg), ctx));
      |                    ~~~^~~~~~
/usr/include/fmt/base.h:1374:21: note: in instantiation of function template specialization 'fmt::detail::value<fmt::context>::format_custom_arg<locator::vnode_effective_replication_map::factory_key, fmt::formatter<locator::vnode_effective_replication_map::factory_key>>' requested here
 1374 |     custom.format = format_custom_arg<
      |                     ^
/home/kefu/dev/scylladb/seastar/include/seastar/util/log.hh:299:33: note: in instantiation of function template specialization 'fmt::format_to<seastar::internal::log_buf::inserter_iterator &, locator::vnode_effective_replication_map::factory_key &, const void *, 0>' requested here
  299 |                     return fmt::format_to(it, fmt.format, std::forward<Args>(args)...);
      |                                 ^
/home/kefu/dev/scylladb/seastar/include/seastar/util/log.hh:428:9: note: in instantiation of function template specialization 'seastar::logger::log<locator::vnode_effective_replication_map::factory_key &, const void *>' requested here
  428 |         log(log_level::debug, std::move(fmt), std::forward<Args>(args)...);
      |         ^
/home/kefu/dev/scylladb/locator/abstract_replication_strategy.cc:561:18: note: in instantiation of function template specialization 'seastar::logger::debug<locator::vnode_effective_replication_map::factory_key &, const void *>' requested here
  561 |         rslogger.debug("create_effective_replication_map: found {} [{}]", key, fmt::ptr(erm.get()));
      |                  ^
/home/kefu/dev/scylladb/locator/abstract_replication_strategy.hh:471:10: note: candidate function template not viable: 'this' argument has type 'const fmt::formatter<locator::vnode_effective_replication_map::factory_key>', but method is not marked const
  471 |     auto format(const locator::vnode_effective_replication_map::factory_key& key, FormatContext& ctx) {
      |          ^
1 error generated.
```

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#19768

locator/abstract_replication_strategy.hh

@@ -468,7 +468,7 @@ struct fmt::formatter<locator::vnode_effective_replication_map::factory_key> {
     }
 
     template <typename FormatContext>
-    auto format(const locator::vnode_effective_replication_map::factory_key& key, FormatContext& ctx) {
+    auto format(const locator::vnode_effective_replication_map::factory_key& key, FormatContext& ctx) const {
         std::ostringstream os;
         os << key;
         return fmt::format_to(ctx.out(), "{}", os.str());