Update test ES 6.8 to use es-oss image

[AndreKurait]

Description

I encountered startup issues running the ES 6.8 image on apple silicon due to x-pack-ml.

Fixed encountered metadata issue when only a single template exists in the cluster.

• Category: Bug Fix
• Why these changes are required? We shouldn't depend on 6.8 x-pack in unit tests
• What is the old behavior before changes and new behavior after changes? Encountered failure org.elasticsearch.bootstrap.StartupException: org.elasticsearch.bootstrap.BootstrapException: java.io.IOException: Cannot run program "/usr/share/elasticsearch/modules/x-pack-ml/platform/linux-x86_64/bin/controller": error=0, Failed to exec spawn helper: pid: 111, exit value: 1 at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:163) exiting container in test

Issues Resolved

MIGRATIONS-2202

Is this a backport? If so, please add backport PR # and/or commits #

Testing

Ran locally on M3 Pro

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.53%. Comparing base (dc326f0) to head (f6c8663).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
...bulkload/version_universal/RemoteReaderClient.java	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1114      +/-   ##
============================================
- Coverage     80.55%   80.53%   -0.03%     
+ Complexity     2930     2929       -1     
============================================
  Files           390      390              
  Lines         14462    14464       +2     
  Branches        998      999       +1     
============================================
- Hits          11650    11648       -2     
- Misses         2209     2211       +2     
- Partials        603      605       +2     

Flag	Coverage Δ
gradle-test	78.60% <60.00%> (-0.03%)	⬇️
python-test	90.33% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.