FlexGrid now accepts columns={1}

[snide]

Summary

EuiFlexGrid now accepts columns={1}. This was at the request of @walterra.

Screen just shows that it's working, I didn't add this example to the docs.

Checklist

• This was checked in mobile
  - [ ] This was checked in IE11
  - [ ] This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
  - [ ] This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
  - [ ] This was checked against keyboard-only and screenreader scenarios

[cchaos]

Code review LGTM