[Actions Plugin] Use server-side authc.getCurrentUser from core.security

[tsullivan]

Part of https://github.com/elastic/kibana/issues/186574

Background: This PR serves as an example of a plugin migrating away from depending on the Security plugin, which is a high priority effort for the last release before 9.0. The Actions plugin uses the authc.getCurrentUser method to attribute the current user to persisted actions that are created in the system.

Checklist

Delete any items that are not applicable to this PR.

• [X] Unit or functional tests were updated or added to match the most common scenarios