[AO] Fix console ":first-child is potentially unsafe" error for the new threshold rule

[maryam-saeidi]

📝 Summary

When creating a new threshold rule, by changing the aggregation, we get this error in the developer console:

The pseudo class ":first-child" is potentially unsafe when doing server-side rendering. Try changing it to ":first-of-type". 
    at Insertion (http://localhost:5601/kibana/9007199254740991/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:161609:20)
    at http://localhost:5601/kibana/9007199254740991/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:161448:73
    at EuiTextColor (http://localhost:5601/kibana/9007199254740991/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:144970:23)
    at EuiText (http://localhost:5601/kibana/9007199254740991/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:144691:24)
    at div
    at ExpressionChart 

✅ Acceptance Criteria

• Fix the error mentioned above in the browser console

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[stephmilovic]

I see this error on page load, on every page. Could be related to Eui changes?

[maryam-saeidi]

Good point!

[maryam-saeidi]

It seems this PR should fix it.

[maryam-saeidi]

It is fixed 🎉