[EuiDataGrid] Reverse asc and desc labels for datetime schema

[thompsongl]

Summary

Closes #4712, in which it was discovered that the sort labels for datetime schema were inverted.

For dates, ascending means that earlier values precede later ones (e.g., 1/1/2020 will sort ahead of 1/1/2021)

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in Chrome, Safari, Edge, and Firefox
- [ ] Props have proper autodocs and playground toggles
- [ ] 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

[myasonik]

I wonder if we do need to change the token names for these string?

Because (I assume) Kibana's i18n process won't know that the string behind these tokens changed (if you change a string in Kibana, you have to manually delete the translations for them to be re-done) so they'll potentially never get updated.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4733/

[thompsongl]

Because (I assume) Kibana's i18n process won't know that the string behind these tokens changed

We'll need to remember to update https://github.com/elastic/kibana/blob/master/src/core/public/i18n/i18n_eui_mapping.tsx with the new labels, but changing the token name isn't necessary.

[chandlerprall]

🦑

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4733/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4733/