Deprecate euiThemeVars

[weronikaolejniczak]

Summary:

The purpose of this ticket is to deprecate euiThemeVars in Kibana.

References:

• Slack thread
• [Meta] EUI Visual Refresh integration and QA kibana#199715 (comment)

[JasonStoltz]

We have a eslint rule with a rule -- we want to make Kibana UI package a warning, but restricted imports is set to error. But can just clone a rule, which is what Weronika did.

Put a PR in for Kibana, but rule is not working.

[JasonStoltz]

2 parts to this:

Get this merged: #8201 -- so Kibana folks can immediately see things shouldn't be used, but CI won't get choked up on it.

Test eslint locally in Kibana to verify it works -- release plugin and bump dependency in Cloud
Lene mentioned that we don't have a good way to deprecate these for server-side usage yet.

We should mark it for deprecation to discourage new usage and get folks to remove their existing usage, but we still need to help folks with server-side usage.