Updated MAINTAINERS.md format.

[dblock]

Coming from opensearch-project/.github#121, updated MAINTAINERS.md to match opensearch-project recommended format.

[dblock]

Looks like some CI has failed. This change has no code changes, can someone please take a look at that?

[saikaranam-amazon]

Failing tests -

• Security plugin is not part of the 3.0 manifest yet

* What went wrong:
Could not determine the dependencies of task ':integTest'.
> Could not resolve all dependencies for configuration ':opensearchPlugin'.
   > Could not resolve org.opensearch.plugin:opensearch-security:3.0.0.0.
     Required by:
         project :
      > Could not resolve org.opensearch.plugin:opensearch-security:3.0.0.0.
         > Could not get resource 'https://d1nvenhzbhpy0q.cloudfront.net/snapshots/lucene/org/opensearch/plugin/opensearch-security/3.0.0.0/opensearch-security-3.0.0.0.pom'.
            > Could not GET 'https://d1nvenhzbhpy0q.cloudfront.net/snapshots/lucene/org/opensearch/plugin/opensearch-security/3.0.0.0/opensearch-security-3.0.0.0.pom'. Received status code [40](https://github.com/opensearch-project/cross-cluster-replication/actions/runs/3849097765/jobs/6557683096#step:4:41)3 from server: Forbidden

• BWC tests - Tracking issue: [BUG] Fix BWC tests on 3.0 #620