Upgrade Gradle IntelliJ plugin to v2

[dennisameling]

I'm trying to run runIde against 2025.2 to debug Kotlin run configurations, but it doesn't work because any version higher than 2024.2 requires major version 2 of the IntelliJ Platform Gradle Plugin:

This PR upgrades said plugin to v2, following the migration guide. Related documentation:

• Blog post: https://blog.jetbrains.com/platform/2024/07/intellij-platform-gradle-plugin-2-0/
• Migration guide: https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin-migration.html
• Sample migration that was advised for reference: https://github.com/JetBrains/intellij-platform-plugin-template/pull/458/files

Can confirm that ./gradlew verifyPlugin passes. Ran runIde with a test project as well, confirming that the env vars from .env are loaded correctly on 2022.3:

I'm happy to bump other versions in follow-up PRs, but didn't want to introduce scope creep here.