Remove unused java11 gradle configuration from lib/core

[andrross]

This is the same unused java11 configuration that was removed from the server module in #11661.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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.

[github-actions]

Compatibility status:

Checks if related components are compatible with change 95ab789

Incompatible components

Incompatible components: [https://github.com/opensearch-project/performance-analyzer.git, https://github.com/opensearch-project/performance-analyzer-rca.git]

Skipped components

Compatible components

Compatible components: [https://github.com/opensearch-project/asynchronous-search.git, https://github.com/opensearch-project/security-analytics.git, https://github.com/opensearch-project/anomaly-detection.git, https://github.com/opensearch-project/observability.git, https://github.com/opensearch-project/reporting.git, https://github.com/opensearch-project/job-scheduler.git, https://github.com/opensearch-project/opensearch-oci-object-storage.git, https://github.com/opensearch-project/custom-codecs.git, https://github.com/opensearch-project/common-utils.git, https://github.com/opensearch-project/notifications.git, https://github.com/opensearch-project/alerting.git, https://github.com/opensearch-project/ml-commons.git, https://github.com/opensearch-project/index-management.git, https://github.com/opensearch-project/neural-search.git, https://github.com/opensearch-project/geospatial.git, https://github.com/opensearch-project/k-nn.git, https://github.com/opensearch-project/cross-cluster-replication.git, https://github.com/opensearch-project/security.git, https://github.com/opensearch-project/sql.git]

[github-actions]

✅ Gradle check result for 95ab789: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a3524b3) 71.45% compared to head (95ab789) 71.26%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #11665      +/-   ##
============================================
- Coverage     71.45%   71.26%   -0.19%     
+ Complexity    59218    59122      -96     
============================================
  Files          4906     4906              
  Lines        278198   278198              
  Branches      40422    40422              
============================================
- Hits         198778   198268     -510     
- Misses        62875    63431     +556     
+ Partials      16545    16499      -46     

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