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 — cs.jdk.version 11 → 21 in root pom.xml
2. CI workflow updates — build.yml JDK 17 → 21; trigger-package-and-publish.yml gets the missing Apache license header and trailing whitespace fix
3. Migration status document — JAVA_21_MIGRATION_STATUS.md with module-by-module analysis, deprecated API inventory (253 hits across 9 categories), and a 6-phase rollout plan

Known blocker (not fixed here, intentionally): cglib-nodep 3.3.0 does not support JDK 21 class files (major version 65). DAO tests in engine/schema fail with Unsupported class file major version 65. The fix (cglib → ByteBuddy migration) is Phase 2 work.

Sandbox build of 25 modules (mvn install -DskipTests -pl server -am) compiles successfully with JDK 21.

Items for reviewer attention

• pom.xml line 58: Verify that cs.jdk.version is only used for maven-compiler-plugin source/target and doesn't have unexpected downstream effects
• argLine (pom.xml line 61): Still contains -noverify which was removed in JDK 13 — may need follow-up
• build.yml: Was previously JDK 17 while pom.xml was JDK 11; this PR aligns both to 21. Build command (-pl server -am) is unchanged
• JAVA_21_MIGRATION_STATUS.md: Deprecated API hit counts are based on regex scanning and are approximate

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:

BUILD FAILURE — cglib blocker (Unsupported class file major version 65):

How Has This Been Tested?

• Sandbox build: mvn install -DskipTests -Dcheckstyle.skip=true -pl server -am with OpenJDK 21 — BUILD SUCCESS (25 modules, ~78 sec)
• Sandbox test: mvn test -pl engine/schema -Dtest=VMInstanceDaoImplTest -Dcheckstyle.skip=true — BUILD FAILURE confirming the cglib blocker (9 errors, 0 failures)
• CI workflows validated via PR checks

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

Ran a targeted DAO test (VMInstanceDaoImplTest) to verify JDK 21 test compatibility, which surfaced the cglib-nodep 3.3.0 incompatibility. This blocker is documented in JAVA_21_MIGRATION_STATUS.md and tracked as Phase 2 work. No application source code was modified.

Link to Devin session: https://app.devin.ai/sessions/6199fc8221124d9b88f85d763000e2ab
Requested by: @hitomimimi