[PERF/ENH] `Series.map` sorts a larger dataset than it needs to

`Series.map` which substitutes values in `self` that match some key with its corresponding value does:
```
lhs = cudf.DataFrame({"x": self, "orig_order": arange(len(self))})
rhs = cudf.DataFrame(
{
"x": arg.keys(),
"s": arg.values(),
"bool": full(len(arg), True, dtype=self.dtype),
}
)
res = lhs.merge(rhs, on="x", how="left").sort_values(
by="orig_order"
)
result = res["s"]
result.name = self.name
result.index = self.index
```

So the order is the same as the input.

This has two pessimisations:

1. In pandas-compat mode (since #14428) this merge doesn't need sorting
2. Since we only return `s`, we can get away with `sort_by_key` of `res["s"]` rather than sorting a multi-column dataframe