[Unified Data Table] Update serializeRowHeight to support number (e…

…lastic#170288)

## Summary

This PR is a cherry-pick to 8.12 from the backport PR elastic#170187 including
a very minor fix for `serializeRowHeight` to support `number` values. I
don't think it has an impact in 8.12 currently, but it will if we
reintroduce the "Reset to default" button in the settings popover.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [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
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/kbn-unified-data-table/src/hooks/use_row_heights_options.ts

@@ -34,6 +34,8 @@ const serializeRowHeight = (rowHeight?: EuiDataGridRowHeightOption): number => {
     return ROWS_HEIGHT_OPTIONS.auto;
   } else if (typeof rowHeight === 'object' && rowHeight.lineCount) {
     return rowHeight.lineCount; // custom
+  } else if (typeof rowHeight === 'number') {
+    return rowHeight;
   }
 
   return ROWS_HEIGHT_OPTIONS.single;