Upgrade to Java 21 and modernize domain models with records

[loesak]

Summary

This PR upgrades the project from Java 11 to Java 21 and modernizes the codebase by replacing Lombok @Value annotations with Java records for all domain models. It also updates dependencies and tooling to leverage modern Java features.

Key Changes

Java Version & Tooling

• Upgraded Java from 11 to 21 across all build configurations (dev container, CI/CD, Maven compiler)
• Updated Maven compiler configuration to use maven.compiler.release property (Java 9+ best practice)
• Added explicit Maven compiler and surefire plugin versions to root pom.xml

Domain Model Modernization

• Converted all domain models to Java records:
  • MigrationRecord - now a record with compact constructor for null validation
  • MigrationLock - now a record with compact constructor for null validation
  • MigrationFile and nested types (MigrationFileMetadata, MigrationFileContents, MigrationFileRequestDefinition) - all converted to records
• Replaced Lombok @Value and @NonNull with Objects.requireNonNull() in compact constructors
• Removed @ConstructorProperties annotations (no longer needed with records)
• Updated all call sites to use record accessor methods (e.g., getMetadata() → metadata())

Dependency Updates

• Elasticsearch client: 7.17.23 → 8.17.0 (now supports Java 21)
• Jackson: 2.17.2 → 2.18.2 (via new jackson-bom for centralized version management)
• SLF4J: 2.0.15 → 2.0.17
• Lombok: 1.18.34 → 1.18.36
• Logback: 1.5.6 → 1.5.16
• Maven plugins: Updated javadoc (3.8.0 → 3.11.2), gpg (3.2.4 → 3.2.7), install/deploy (3.1.2 → 3.1.3)

Code Quality Improvements

• Replaced Collectors.toUnmodifiableList() with Stream.toList() (Java 16+)
• Removed unnecessary type casts in exception handling
• Updated documentation to reflect Java 21 and Elasticsearch 8.x support
• Removed TODO about building separate ES 7.x/8.x artifacts (now unified on 8.x)

Documentation Updates

• Updated CLAUDE.md to reflect Java 21 requirement and ES 8.x client version
• Updated dev container description to reference JDK 21
• Updated architecture documentation to note that domain models are now records

Implementation Details

• Records provide immutability and reduce boilerplate compared to Lombok @Value
• Compact constructors in records handle null validation via Objects.requireNonNull()
• Jackson 2.18+ has native record deserialization support, eliminating need for @ConstructorProperties
• Jackson BOM import in root pom.xml ensures consistent versions across all Jackson modules
• All existing functionality preserved; this is a modernization refactor with no behavioral changes

https://claude.ai/code/session_015ESZFHmSSx9fBhLHb5EYoD