Kibana 7.7.0: Console warnings when adding a color rule to a TSVB Metric visualization

[Ron-70]

Kibana version: 7.7.0

Elasticsearch version: 7.7.0

Server OS version: Windows 7 Enterprise

Browser version: Firefox 76.0.1

Browser OS version: Windows 7 Enterprise

Describe the bug: Adding a color rule to a TSVB Metric visualization causes a warning in Kibana console log.

Steps to reproduce:

1. Create a new TSVB Visualization
2. Select Metric Tab
3. Panel options --> Choose a color rule from the select box (e.q. > greater than)
4. Kibana console warning appears

Expected behavior: No warnings

Errors in kibana console:
log [06:02:46.739] [warning][metrics][plugins][visTypeTimeseries] Request validation error: child "panels" fails because ["panels" at position 0 fails because [child "background_color_rules" fails because ["background_color_rules" at position 0 fails because ["operator" is not allowed]]]] (saved object id: unsaved). This most likely means your TSVB visualization contains outdated configuration. You can report this problem under https://github.com/elastic/kibana/issues/new?template=Bug_report.md

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)