Fixes CODEOWNERS by putting all on a single line. Updated MAINTAINERS.

[dlvenable]

Description

Fixes the CODEOWNERS file by putting all users on a single line. Also, moves Shivani to Emeritus section.

See: https://github.com/opensearch-project/logstash-output-opensearch/pull/203#discussion_r1142562128

Issues Resolved

N/A

Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has documentation added
• [ ] Commits are signed as per the DCO using --signoff

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