[Backport 2.x] Update CI to use jdk-21

[ryanbogan]

Description

This updates the CI system to use jdk-21, which is latest LTS supported version.
Coming from opensearch-project/OpenSearch#10334

Backport of #599

Check List

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (bc355fd) 89.03% compared to head (b2aeb31) 89.03%.
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #604   +/-   ##
=========================================
  Coverage     89.03%   89.03%           
  Complexity      767      767           
=========================================
  Files            93       93           
  Lines          2746     2746           
  Branches        223      223           
=========================================
  Hits           2445     2445           
  Misses          221      221           
  Partials         80       80           

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