[ESLint] Warn about static z-index

[weronikaolejniczak]

Summary

Testing local version of @elastic/eslint-plugin-eui: elastic/eui#9236

[elasticmachine]

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[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 2622054

Failed CI Steps

• FTR Configs #89
• FTR Configs #101

Test Failures

• [job] [logs] FTR Configs #101 / Alerting builtin alertTypes es_query rule runs correctly: use epoch millis - threshold on hit count < > for esQuery search type
• [job] [logs] FTR Configs #89 / ObservabilityApp Custom threshold rule with ad-hoc data view should create a custom threshold rule with an ad-hoc data view

Metrics [docs]

✅ unchanged