[Security Solution]Customize event rendering under timeline area was disabled on all browser

Describe the bug
customize event rendering under timeline area was disabled on all browser

Build Details:

Platform: Production
Version: 7.12.0 BC2
Commit: 4f65a5a1268fa78f1af9117d12312e1cee433376
Build:39000

Browser Details

Edge : 88.0.705.74 
Safari: 13.1.2 [13609.3.5.1.5]
Firefox: 86.0 
Chrome: 88.0.4324.190 

Preconditions
1.Elastic Cloud environment having version 7.12.0 should be available.
2.Audit beat should be installed on above environment.

Steps to Reproduce

1. Navigate to Kibana and Select Security Solution App from the left navigation panel.
2. Go to Timeline Tab.
3. Enter any filter Query "host.name: "*" " in the timeline search bar.
4. Click on customize event rendering button.
5. Observed the Customize event rendering page was disabled for user to perform any action.

Actual Result
Customize event rendering under timeline area was disabled on all browser

Expected Result
Customize event rendering under timeline area should open correctly and accessible to user.

Whats Working

• N/A

Whats Not Working

• N/A

Screenshots

logs
N/A

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

Assigned to @MadameSheema

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)

[MadameSheema]

Found during BC2 regression https://github.com/elastic/security-team/issues/843

[andrew-goldstein]

This issue is the result of a breaking change in EUI, via this PR: elastic/eui#4480

The behavior of EuiModal has changed such that it's no longer necessary to wrap the modal in an EuiOverlayMask, because the mask is now built-in to EuiModal.

The breaking change became effective throughout Kibana when it was upgraded to use a newer version of EUI via this commit on Feb 16.

I'll put up a PR today that removes the now-redundant EuiOverlayMask, which will resolve the issue.