Default discovery-ec2 region to none.

[dblock]

Description

Defaults discovery-ec2 plugin default region to none. According to https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/region-selection.html this should automatically determine the Region from the environment.

Related Issues

Resolves #7959.

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
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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[dblock]

@raghuvanshraj do we need to do something similar for repository-s3?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.backpressure.SearchBackpressureIT.testSearchShardTaskCancellationWithHighCpu
      1 org.opensearch.remotestore.RemoteStoreRefreshListenerIT.testRemoteRefreshRetryOnFailure

• URL: https://build.ci.opensearch.org/job/gradle-check/17145/
• CommitID: 09de05f
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Merging #7969 (09de05f) into main (c888fce) will increase coverage by 0.02%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##               main    #7969      +/-   ##
============================================
+ Coverage     70.84%   70.87%   +0.02%     
+ Complexity    56464    56437      -27     
============================================
  Files          4702     4702              
  Lines        266973   266975       +2     
  Branches      39157    39158       +1     
============================================
+ Hits         189131   189207      +76     
+ Misses        61897    61808      -89     
- Partials      15945    15960      +15     

Impacted Files	Coverage Δ
...rg/opensearch/discovery/ec2/AwsEc2ServiceImpl.java	89.06% <75.00%> (-1.27%)	⬇️
...rg/opensearch/discovery/ec2/Ec2ClientSettings.java	98.50% <100.00%> (ø)

... and 469 files with indirect coverage changes

[raghuvanshraj]

@raghuvanshraj do we need to do something similar for repository-s3?

I think this might be causing the failure in YAML rest tests and third party tests as well. Ref: #7866
I'll test out similar changes for repository-s3.