Bump min Gradle requirement to 9.4.0

[Goooler]

Aims for release 9.7.0.

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

[Copilot]

Pull request overview

This PR raises Shadow’s minimum supported Gradle version to 9.4.0, allowing removal of pre-9.4 compatibility shims and simplifying related functional tests, CI matrix, and documentation.

Changes:

• Bump the minimum Gradle version used for compatibility testing/CI to 9.4.0.
• Remove Gradle < 9.4 compatibility helpers (extendsFromCompat, moveGradleApiIntoCompileOnly) and simplify plugin code accordingly.
• Update docs and changelog to reflect the new minimum requirement.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowJavaPlugin.kt	Switches configuration wiring to direct extendsFrom(...) and removes legacy Gradle API relocation hook.
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/GradleCompat.kt	Deletes Gradle < 9.4 compatibility helpers no longer needed after the version bump.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/JavaPluginsTest.kt	Simplifies assertions now that Gradle < 9.4 is no longer supported in the test matrix.
README.md	Updates the compatibility table to reflect the new minimum Gradle version for the next release line.
docs/README.md	Mirrors the README compatibility table update for the documentation site.
docs/changes/README.md	Adds an Unreleased changelog entry for the minimum Gradle bump.
build.gradle.kts	Updates Tapmoc’s minimum Gradle configuration to 9.4.0.
.github/workflows/build.yml	Updates CI to test against Gradle 9.4.0 and current.

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