Update generator to Kotlin 2.4.20 and KotlinPoet 2.3.0

[ExNDY]

Summary

Updates the moko-resources generator toolchain while preserving the existing public compatibility baseline.

Changes

• Updated generator Kotlin from 2.3.20 to 2.4.20.
• Updated KotlinPoet from 1.13.1 to 2.3.0.
• Kept Kotlin 2.1.0 as the minimum supported version for public modules.
• Set the generator language and API versions to Kotlin 2.1.
• Kept AGP 8.13.2 as the generator compile-time API while supporting AGP 8.x and AGP 9.x consumers.
• Replaced the removed internal Kotlin/Native zipDirAs API with a local deterministic ZIP implementation.
• Added tests for deterministic KLIB ZIP generation, entry ordering, file contents, and empty directories.
• Added a dynamic KDoc index to generated MR objects with links to available resource containers.
• Moved the compatibility policy to COMPATIBILITY.md.
• Reduced README.md to the public requirements and a link to the compatibility policy.
• Documented why KotlinPoet 2.4.0 is intentionally not used yet.

Example generated documentation:

/**
 * Entry point for generated resources.
 *
 * Available resource containers:
 * - [strings]
 * - [plurals]
 * - [images]
 * - [colors]
 * - [fonts]
 * - [files]
 * - [assets]
 */
public expect object MR

Compatibility

• Public Kotlin baseline remains 2.1.0.
• Public AGP baseline remains 8.3.0.
• Generator Kotlin toolchain uses 2.4.20.
• Generator KotlinPoet remains on 2.3.0 because 2.4.0 does not provide an API or fix currently required by the generator.
• Kotlin baseline 2.2.0 may be adopted after Kotlin 2.5 stabilization, or earlier if Kotlin 2.5 compatibility fixes cannot be released while retaining Kotlin 2.1 support.
• AGP 9 compile API migration remains a separate compatibility change.