[EuiOverlayMask] Remove `onClick` in favor of `EuiFocusTrap` combo

[thompsongl]

EuiOverlayMask <-- we've had issues in the past with click events on our overlay mask that Greg had to fix, and I'm a little worried about this potentially reintroducing bugs.

Willing to hear arguments against, but I think that EuiOverlayMask should no longer accept an onClick prop. The intention was to give an easy on-outside-click hook, but this is now inconsistent with our other approaches to outside click behavior, which have largely been standardized.

If it's necessary to unblock your React 17 PR, we can add the isCapture prop or prioritize the breaking change to remove onClick.

The suggestion would then be to use EuiFocusTrap in conjunction with EuiOverlayMask as we do in several components (e.g., EuiFlyout and EuiCodeBlock):

<EuiOverlayMask>
  <EuiFocusTrap onClickOutside={onClick} />
</EuiOverlayMask>

Originally posted by @thompsongl in #5940 (comment)