[EuiComboBox] Fix keyboard selection when sortMatchesBy=startsWith

[thompsongl]

Summary

Fixes #3812, in which using the keyboard to select an option in EuiComboBox when sortMatchesBy=startsWith would select the wrong option.

The problem arose from the sort logic only occurring during the render function, bypassing the state store that the keyboard logic uses to track active options.

This change moves the logic to the central getMatchingOptions function, which updates component state.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation

• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3823/

[chandlerprall]

Changes LGTM! Tested with the published docs' Sorting matches example, where the bug has been reproducible. Also tested examples without startsWith to verify those still function as expected.

Thanks for adding the test case!

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3823/