Description

This PR establishes the foundation for migrating Apache CloudStack from JDK 11 to JDK 21. It contains three atomic changes:

1. JDK version bump (pom.xml): cs.jdk.version changed from 11 to 21.
2. CI workflow updates (.github/workflows/):
   • build.yml: JDK 17 → 21 in the setup-java step.
   • trigger-package-and-publish.yml: Added missing Apache License header; fixed trailing whitespace on the chmod line.
3. Migration status document (JAVA_21_MIGRATION_STATUS.md): Module-by-module inventory (7,597+ Java files across 11 modules), deprecated API scan (263 hits across 9 categories), known blockers, effort estimates (12–18 dev-days), and a 6-phase rollout plan.

No application source code is modified. The build command in build.yml (mvn -B -P developer,systemvm -Dsimulator -DskipTests install) is intentionally unchanged.

Sandbox verification: 25 modules compiled successfully with OpenJDK 21.0.10 via mvn install -DskipTests -Dcheckstyle.skip=true -pl server -am.

Known blocker identified: cglib-nodep:3.3.0 is a compile dependency in engine/schema (via Spring). While Mockito-based unit tests pass (using ByteBuddy), cglib 3.3.0 is unmaintained (last release 2019) and poses a risk for Spring CGLIB proxy generation with JDK 21 class files (major version 65). A follow-up PR is needed to migrate to ByteBuddy.

Items for reviewer attention

• pom.xml argLine (not changed in this PR): The existing flags include -noverify (ignored since JDK 13) and several --add-opens/--add-exports. These may need revisiting for JDK 21 in a follow-up.
• Migration doc numbers: The deprecated API total is 263 from the combined regex scan, while the per-module column sums to 253. The difference comes from files in directories outside the listed modules (e.g., test/, developer/). Both numbers are from production code only (test directories excluded from scan).
• Other CI workflows (ci.yml, codecov.yml, sonar-check.yml) still reference JDK 17 but are gated by github.repository == 'apache/cloudstack' and won't execute on this fork.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• Build/CI
• Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Screenshots (if appropriate):

BUILD SUCCESS — 25 modules compiled with JDK 21:

Unit test verification — engine/schema VMInstanceDaoImplTest (9/9 passed):

How Has This Been Tested?

• Sandbox build: mvn install -DskipTests -Dcheckstyle.skip=true -pl server -am with OpenJDK 21.0.10 — BUILD SUCCESS (25 modules, ~64s).
• Unit test: mvn test -pl engine/schema -Dtest=VMInstanceDaoImplTest -Dcheckstyle.skip=true — 9 tests passed.
• CI: GitHub Actions build.yml workflow with JDK 21.

How did you try to break this feature and the system with this change?

• Ran the cglib-dependent DAO test (VMInstanceDaoImplTest) to verify whether cglib 3.3.0 causes Unsupported class file major version 65 errors. Tests passed (Mockito uses ByteBuddy), but cglib remains a compile dependency and a runtime risk — documented in the migration status.

Link to Devin session: https://app.devin.ai/sessions/ade9c90352034c45b5fd8251bd45eade
Requested by: @hitomimimi