Replace uses of deprecated TransportClient

[davidlago]

Describe the bug
After the deprecated TransportClient was removed in 2.0, we need to go through the uses in this repo and update the to use a REST client instead.

Current behavior
Building the security plugin against OpenSearch 2.0 gives the following error:

Error:  /home/runner/work/security/security/src/main/java/org/opensearch/security/tools/SecurityAdmin.java:[92,39] cannot find symbol
Error:    symbol:   class TransportClient
Error:    location: package org.opensearch.client.transport

Acceptance criteria
Security plugin builds against OpenSearch 2.0 with no errors, test pass.