QUARKUS-2994|JDK 21 support

QUARKUS-2994.md

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+# QUARKUS-2994 JDK 21 Support
+
+This document is a high level test plan for certification of Red Hat build of Quarkus on JDK 21.
+
+## Scope of the testing
+
+### Current test coverage
+* Regular CI runs in upstream include full testing for Java 21 (according to the announcement)
+* Daily runs on java 21 in GitHub Actions
+* Checks of code.quarkus in StartStop TS
+
+### Impact on test suites and testing automation
+* Temurin JVM 21 and OpenJDK 21 need to be added as JDKs to Quarkus QE Jenkins
+* Temurin JVM 21 and OpenJDK 21 have to be added as JVMs to Polarion
+* Bare metal jobs in extended platform testing need to be extended to run with both Temurin 21 and OpenJDK 21.
+  * This affects JVM mode only.
+* Openshift JVm job needs to use OpenJDK 21 (together with existing 17) instead of OpenJDK 11
+  * We also should use s2i build image based on jdk 21 there
+* Two quickstart jobs (Linux and Windows) should be added/repurposed from 11 in general or extended pipeline 
+
+### Impact on resources
+* Should not change (or be minimal), since for affected jobs we should drop testing on java 11, so consumed resource will stay the same.
+* In general support for JDK depends on number of used jobs. For basic product support, each run requires the following resources on Openstack:
+  *  Baremetal 3 combinations (podman, docker, fips) * 8 scenarios * `large`(8192 RAM + 4 vCPU) node
+  *  openshift two combinations (eus, stable) * `medium`(4096 RAM + 4 vCPU) node.
+    * Each combination includes ~180 tests (running together for 40 minutes), each creates and deletes a dedicated Openshift namespace of unknown resource consumption  
+  * Quickstarts on RHEL: `large` jenkins node (see above)
+  * Quickstarts on Windows: `medium` jenkins node and `m5zn.metal` amazon EC2 instance
+
+## Future considerations
+* Since updating JDKs require additional effort on both ours and infra team side, we may have to come up with setting up the JDK ourselves
+  * If snapshot-local setup is used, testing snapshots have to be respun every time a release is made(but we do it every two weeks anyway)
+  * If we install JDK as part of the testing job, this will make using matrix jobs very inconvenient
+
+## Contacts
+* Tester: Fedor Dudinsky <[email protected]>
+
+## References
+* Feature epic: [QUARKUS-2994 JDK 21 Support](https://issues.redhat.com/browse/QUARKUS-2994)
+* Temurin support TP: [QUARKUS-1936](QUARKUS-1936.md)
+* Upstream support announcement: https://quarkus.io/blog/quarkus-3-5-0-released/