Support IntelliJ Platform Gradle Plugin 2

[FinlayRJW]

Before this PR

gradle-external-publish-plugin 2.0.0 uses IntelliJ Platform Gradle Plugin 2.x. This repository still uses its removed 1.x DSL and cannot consume the release. JetBrains documents the required changes in the migration guide.

After this PR

==COMMIT_MSG==
==COMMIT_MSG==

The build consumes gradle-external-publish-plugin 2.0.0 and uses the IntelliJ Platform 2.x DSL. Existing IntelliJ versions, compatibility bounds, metadata, bundled plugins, and verifier targets remain unchanged.

Testing

• The IntelliJ plugin tests pass.
• buildPlugin passes.
• Formatting, Error Prone, and Checkstyle checks pass.

Possible downsides?

None known.

Are Docs needed?

No.

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Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

Support IntelliJ Platform Gradle Plugin 2

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💡 Improvements

• Support IntelliJ Platform Gradle Plugin 2 (#1056)