crimson/osd: s/seastar::fprint()/fmt::print()/

crimson/osd: s/seastar::fprint()/fmt::print()/

otherwise, we'd have warnings like:

./src/crimson/osd/main.cc:106:16: error: 'fprint<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > &>' is deprecated: use std::format_to() or fmt::print() [-Werror,-Wdeprecated-declarations]
      seastar::fprint(std::cerr, "already have key in keyring: %s\n", path);
               ^

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