[BETA] High Contrast Mode

### Overview

`High Contrast Mode` originates from a feature request: https://github.com/elastic/eui/issues/8133. It has been introduced on this PR: https://github.com/elastic/eui/pull/8444 and is currently in **beta**.

We introduced High Contrast Mode with the intent of integrating it into Kibana in order to address accessibility concerns with contrast. It is a means of offering high contrast without a large visual change to the core EUI theme.

We introduced it fully expecting that there will be additional requirements that we missed on first introduction. It will remain in Beta until it has reached a point where it fully fulfills the accessibility needs of Kibana.

This issue is intended to track the progress and necessary actions to transition the functionality out of beta status and into general availability.

### Feature Description

High contrast mode is documented in [our docs](https://eui.elastic.co/#/theming/high-contrast-mode).

### Next Steps

To facilitate the transition, we should:

- [ ] Monitor and service incoming feature requests and bugs for High Contrast Mode.
- [ ] Work with the @elastic/appex-sharedux team to determine whether or not High Contrast Mode now offers the level of support they need to consider it GA.

### Additional Context (optional)

Adjustments to High Contrast so far:
- https://github.com/elastic/eui/pull/8550
- https://github.com/elastic/eui/pull/8496