utils: estimated_histogram: convert generated format strings to fmt
utils: estimated_histogram: convert generated format strings to fmt

Convert printf games to format games.

Note that fmt supports specifying the field width as an argument, but that
is left to a dedicated change.

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla