Update build system to Gradle 9.2.1 and Java 21

[ryan-rushton]

Summary

This PR updates build tooling, dependencies, and applies automated formatting from ktlint updates.

Changes

Build System & Dependencies

• Upgraded to Gradle 9.2.1
• Updated IntelliJ Platform Gradle Plugin to 2.10.5
• Updated ktlint to 14.0.1 (triggered repo-wide formatting)
• Updated Kotlin to 2.3.0
• Updated various dependency versions (Kotest 6.0.7, etc.)

Test Configuration

• Fixed Kotest integration with JUnit 5 by adding testRuntimeOnly("junit:junit:4.13.2")
• Ensures tests run properly with Gradle test infrastructure without IntelliJ Platform test framework conflicts

CI/CD Updates

• Modernized GitHub Actions workflows
• Updated action versions (checkout@v5, setup-java@v5, setup-gradle@v5, upload-artifact@v5)
• Added disk space optimization for plugin verifier
• Simplified release workflow

Code Formatting

• Applied ktlint formatting updates across all Kotlin source and test files
• No functional changes to source code

Documentation

• Updated CHANGELOG.md with build system improvements
• Updated CLAUDE.md to reflect Java 21 requirement and IntelliJ 2025.1+ target
• Updated README.md to specify IntelliJ 2025.1+ compatibility