fix: Return appropriate data type for duration mean and median

[mcrumiller]

Resolves #14375. Part of overall #13599.

This also moves one step closer to deprecating Series.dt.mean and Series.dt.median, as the base Series' mean and median now support duration dtypes.

Using the example from the issue, all cases now properly output a duration:

6 days, 0:00:00
2 days, 0:00:00
6 days, 0:00:00
2 days, 0:00:00
shape: (1, 2)
┌──────────────┬──────────────┐
│ mean         ┆ median       │
│ ---          ┆ ---          │
│ duration[μs] ┆ duration[μs] │
╞══════════════╪══════════════╡
│ 6d           ┆ 2d           │
└──────────────┴──────────────┘
shape: (1, 3)
┌─────┬──────────────┬──────────────┐
│ key ┆ mean         ┆ median       │
│ --- ┆ ---          ┆ ---          │
│ i64 ┆ duration[μs] ┆ duration[μs] │
╞═════╪══════════════╪══════════════╡
│ 1   ┆ 6d           ┆ 2d           │
└─────┴──────────────┴──────────────┘