[EuiComboBox] Update with panelMinWidth behavior

[cee-chen]

See #7071

EuiComboBox needs to be updated with this same UX. The component is not using EuiInputPopover under the hood, for whatever reason, and thus does not inherit the above functionality. We need to fix this in one of two ways:

• Update the component to dogfood EuiInputPopover
• If there's a specific reason the component can't use EuiInputPopover, update its popover width logic with similar behavior and add a custom prop