Bump min Gradle requirement to 9.0.0

[Goooler]

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• CHANGELOG's "Unreleased" section has been updated, if applicable.

[Copilot]

Pull Request Overview

Updates the minimum Gradle requirement from 8.11 to 9.0.0 for the Shadow plugin version 9.3.0+.

• Bumped minimum Gradle version requirement to 9.0.0 in documentation and CI configuration
• Updated string interpolation syntax in test files to use dollar-dollar syntax ($$) required by newer Gradle versions
• Simplified CI matrix by removing Java 24 exclusion that was specific to older Gradle versions

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/README.md	Added new row for version 9.3.0+ with Gradle 9.0 minimum requirement
README.md	Added new row for version 9.3.0+ with Gradle 9.0 minimum requirement
.github/workflows/build.yml	Updated CI matrix to use Gradle 9.0.0 as minimum and removed Java 24 exclusion
src/test/kotlin/.../SimpleRelocatorTest.kt	Updated string interpolation syntax from \$ to $$ for Gradle compatibility
src/functionalTest/kotlin/.../JavaPluginsTest.kt	Updated string interpolation syntax from \$ to $$ for Gradle compatibility
src/functionalTest/kotlin/.../ApplicationPluginTest.kt	Updated string interpolation syntax from \$ to $$ for Gradle compatibility

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