cmake: require 4.0.0 <= libfmt-dev < 5.0.0

cmake: require 4.0.0 <= libfmt-dev < 5.0.0

FMT_VERSION is now living in fmt/core.h in libfmt >= 5.0, so we cannot
parse fmt/format.h for libfmt's version anymore. also we don't need
Findfmt.cmake now. because libfmt-dev 4.0 and up is required by seastar,
also it's libfmt-dev 3.x's cmake module which fails to offer fmt::fmt
target. in other words, it's safe to drop libfmt 3.x support. but 5.0.0
is not compatible with seastar, we will have failures like:

In file included from
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/aligned_buffer.hh:25:0,
                 from
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/reactor.hh:26,
                 from
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/alien.hh:35,
                 from
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/src/core/alien.cc:23:
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/print.hh:
In function 'seastar::sstring seastar::format(const char*, A&& ...)':
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/print.hh:135:10:
error: 'MemoryWriter' is not a member of 'fmt'
     fmt::MemoryWriter out;
          ^~~~~~~~~~~~
/home/smithfarm/src/ceph/smithfarm/ceph/src/seastar/include/seastar/core/print.hh:136:5:
error: 'out' was not declared in this scope
     out.write(fmt, std::forward<A>(a)...);
     ^~~

so, we should build libfmt even if we have libfmt-dev 5.x

Signed-off-by: Kefu Chai <kchai@redhat.com>