Closes elastic#211930

## Summary

This PR makes it so that `kbn-grid-layout` stores its rows as an object
/ dictionary (`{ [key: string]: GridRowData }`) rather than an array
(`Array<GridRowData>`). This is a prerequisite for
elastic#190381 , since it allows us to
re-order rows without re-rendering their contents. It also means that
deleting a row will no longer cause the rows below it to re-render,
since re-rendering is now dependant on the row's **ID** rather than the
row's order.

**Before**

https://github.com/user-attachments/assets/83651b24-a32c-4953-8ad5-c0eced163eb5

**After**

https://github.com/user-attachments/assets/9cef6dbc-3d62-46aa-bc40-ab24fc4e5556

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit b32f0fe)

… (#213162)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[kbn-grid-layout] Store rows in object instead of array
(#212965)](#212965)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

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Store rows in object instead of array (#212965)\n\nCloses
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Store rows in object instead of
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Store rows in object instead of array (#212965)\n\nCloses
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makes it so that `kbn-grid-layout` stores its rows as an object\n/
dictionary (`{ [key: string]: GridRowData }`) rather than an
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correct `release_note:*` label is applied per
the\n[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)\n\n---------\n\nCo-authored-by:
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Store rows in object instead of array (#212965)\n\nCloses
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makes it so that `kbn-grid-layout` stores its rows as an object\n/
dictionary (`{ [key: string]: GridRowData }`) rather than an
array\n(`Array<GridRowData>`). This is a prerequisite
for\nhttps://github.com//issues/190381 , since it allows
us to\nre-order rows without re-rendering their contents. It also means
that\ndeleting a row will no longer cause the rows below it to
re-render,\nsince re-rendering is now dependant on the row's **ID**
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correct `release_note:*` label is applied per
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BACKPORT-->

Co-authored-by: Hannah Mudge <Heenawter@users.noreply.github.com>