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>
Closesscylladb/scylladb#19768
db56af2e41