DEPR: change Panel DeprecationWarning -> FutureWarning
closes #18324

yeah I'll maybe make a sub-section essentially repeating the original whatsnew code. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=h1) Report
> Merging [#19247](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aa9e002463d104b6dc37cdb32b2726bafcb5aa36?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19247/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19247   +/-   ##
=======================================
  Coverage   91.52%   91.52%           
=======================================
  Files         147      147           
  Lines       48827    48827           
=======================================
  Hits        44691    44691           
  Misses       4136     4136
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <ø> (ø)` | :arrow_up: |
| #single | `41.69% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/19247/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `96.83% <ø> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=footer). Last update [aa9e002...4d1194d](https://codecov.io/gh/pandas-dev/pandas/pull/19247?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
