Timex.shift/2 with time-shift always converts precision to :microsecond in Elixir v1.14+

### Steps to reproduce

with Elixir v1.13
```elixir
iex(1)> now = ~U[2023-01-01 10:20:30Z]
~U[2023-01-01 10:20:30Z]
iex(2)> Timex.shift(now, hours: 1)
~U[2023-01-01 11:20:30Z] # OK
```

with Elixir v1.14
```elixir
iex(1)> now = ~U[2023-01-01 10:20:30Z]
~U[2023-01-01 10:20:30Z]
iex(2)> Timex.shift(now, hours: 1)
~U[2023-01-01 11:20:30.000000Z] # precision changes unnecessarily
```

### Description of issue

- Elixir v1.14 changes DateTime.add/4 behaviour to preserve the maximum precision of given parameters.
- commit: https://github.com/elixir-lang/elixir/commit/5a583c753b96865a7cdec2fb4c1ab9c96b836d24
- relates to: https://github.com/elixir-lang/elixir/issues/12303
- Timex.shift/2 uses DateTime.add/4 with `:microsecond` precision to apply time-shift result.
- https://github.com/bitwalker/timex/blob/e1c4d424a5c7840f63efd33cd70e3b5d4f9bfaa1/lib/datetime/datetime.ex#L450

This causes unnecessary precision conversions in Elixir v1.14 and later.