ki: update to use jdk21 and deprecate

[chenrui333]

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not compatible with kotlin 2.0+

• Incompatible with Kotlin 2.0 Kotlin/kotlin-interactive-shell#131, https://endoflife.date/kotlin
• openjdk: 25.0.1 #250760

[Copilot]

Pull Request Overview

This PR updates the ki formula to use JDK 21 and marks the formula as deprecated due to incompatibility with Kotlin 2.0+. The changes ensure the formula continues to work with JDK 21 while warning users that the project is unmaintained.

Key Changes:

• Added deprecation notice with reference to upstream issue
• Updated dependency from openjdk to openjdk@21
• Modified all Java-related configuration to use JDK 21 specifically

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[github-actions]

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