fix(polars): swap out pivot for pivot_stable to suppress warning message (#15913)

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# Description
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The current implementation of `polars pivot` calls an unsupported
version of pivot that throws a warning message in stdout (using
println!) stating that "unstable pivot not yet unsupported, using stable
pivot." This PR swaps out the call to `pivot` with a call to
`pivot_stable`, which is being done in the underlying polars anyways.

```nushell
#  Current Implementation
> [[a b c]; [1 x 10] [1 y 10] [2 x 11] [2 y 11]] | polars into-df | polars pivot -i [a] -o [b] -v [c]
unstable pivot not yet supported, using stable pivot
╭───┬───┬────┬────╮
│ # │ a │ x  │ y  │
├───┼───┼────┼────┤
│ 0 │ 1 │ 10 │ 10 │
│ 1 │ 2 │ 11 │ 11 │

#  Proposed Implementation (no println! statement)
> [[a b c]; [1 x 10] [1 y 10] [2 x 11] [2 y 11]] | polars into-df | polars pivot -i [a] -o [b] -v [c]
╭───┬───┬────┬────╮
│ # │ a │ x  │ y  │
├───┼───┼────┼────┤
│ 0 │ 1 │ 10 │ 10 │
│ 1 │ 2 │ 11 │ 11 │
╰───┴───┴────┴────╯
```

# User-Facing Changes
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None

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Current suite of tests were sufficient

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crates/nu_plugin_polars/src/dataframe/command/data/pivot.rs

@@ -5,7 +5,7 @@ use nu_protocol::{
 };
 
 use chrono::DateTime;
-use polars_ops::pivot::{PivotAgg, pivot};
+use polars_ops::pivot::{PivotAgg, pivot_stable};
 
 use crate::{
     PolarsPlugin,
@@ -237,7 +237,7 @@ fn command_eager(
 
     let polars_df = df.to_polars();
     // todo add other args
-    let pivoted = pivot(
+    let pivoted = pivot_stable(
         &polars_df,
         &on_col_string,
         Some(&index_col_string),