Updated MAINTAINERS.md format.

[dblock]

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

[codecov-commenter]

Codecov Report

Merging #734 (32880e4) into main (6f48746) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #734      +/-   ##
============================================
- Coverage     75.46%   75.31%   -0.15%     
  Complexity      116      116              
============================================
  Files           125      125              
  Lines          6822     6822              
  Branches       1021     1021              
============================================
- Hits           5148     5138      -10     
- Misses         1142     1153      +11     
+ Partials        532      531       -1     

Impacted Files	Coverage Δ
.../alerting/transport/TransportIndexMonitorAction.kt	54.93% <0.00%> (-2.97%)	⬇️
.../kotlin/org/opensearch/alerting/util/IndexUtils.kt	70.83% <0.00%> (-2.09%)	⬇️
...rch/alerting/opensearchapi/OpenSearchExtensions.kt	50.00% <0.00%> (-1.62%)	⬇️
...destinationmigration/DestinationConversionUtils.kt	70.00% <0.00%> (ø)
...ing/model/destination/DestinationContextFactory.kt	78.57% <0.00%> (+3.57%)	⬆️

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[dblock]

@lezzago Can we get a second PR here and merge this pls?