Increment version to 2.9 and refactor

[monusingh-1]

Description

commit on 2.x opensearch-project/OpenSearch#8316
commit on main opensearch-project/OpenSearch#7309
Fix build failures from upstream changes

Replaced existing uses of import java.util.* to relevant imports
Removed uses of ImmutableOpenMap and changed to Map
Added Null safety

Opensearch removed the remaining uses of ImmutableOpenMap to Map.
the get() of ImmutableOpenMap return non-null empty set however the get() method of Map can return null. To avoid this we changed the uses of get() to getOrDefault() to handle null cases gracefully.

Ref #814, #837

Issues Resolved

#1005

Check List

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

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[codecov]

Codecov Report

Merging #1061 (0a84967) into 2.9 (b98d384) will increase coverage by 0.63%.
The diff coverage is 48.38%.

❗ Current head 0a84967 differs from pull request most recent head 2a9f092. Consider uploading reports for the commit 2a9f092 to get more accurate results

@@             Coverage Diff              @@
##                2.9    #1061      +/-   ##
============================================
+ Coverage     72.65%   73.28%   +0.63%     
- Complexity     1020     1023       +3     
============================================
  Files           141      141              
  Lines          4735     4736       +1     
  Branches        528      540      +12     
============================================
+ Hits           3440     3471      +31     
+ Misses          962      927      -35     
- Partials        333      338       +5     

Impacted Files	Coverage Δ
...on/resume/TransportResumeIndexReplicationAction.kt	85.29% <0.00%> (-4.42%)	⬇️
...lication/action/status/TranportShardsInfoAction.kt	96.66% <ø> (ø)
...ion/action/update/UpdateIndexReplicationRequest.kt	50.00% <ø> (ø)
.../replication/metadata/store/ReplicationMetadata.kt	62.96% <ø> (ø)
.../opensearch/replication/rest/UpdateIndexHandler.kt	100.00% <ø> (ø)
...ication/seqno/RemoteClusterRetentionLeaseHelper.kt	70.43% <0.00%> (-1.74%)	⬇️
...rch/replication/task/index/IndexReplicationTask.kt	67.72% <38.88%> (-0.99%)	⬇️
...lication/metadata/TransportUpdateMetadataAction.kt	76.36% <100.00%> (ø)
...earch/replication/metadata/UpdateIndexBlockTask.kt	74.19% <100.00%> (ø)
.../replication/repository/RemoteClusterRepository.kt	72.99% <100.00%> (ø)

... and 4 files with indirect coverage changes