Upgrade Gradle from 8.5 to 9.7.1

[madsboddum]

Upgrades the Gradle wrapper from 8.5 to 9.7.1. The wrapper files (gradlew, gradlew.bat, the wrapper jar and properties) are regenerated by ./gradlew wrapper --gradle-version 9.7.1.

Four companion upgrades were required to get a working build:

• Kotlin plugin 2.1.21 -> 2.4.10. The build derives its JVM target from the running JDK, and 2.1.x has no JVM target 26.
• org.beryx.jlink 3.1.1 -> 4.1.1 for Gradle 9 compatibility.
• archunit-junit5 1.3.0 -> 1.5.0. 1.3.0 cannot read class file major version 70, so it imported no classes and every ArchUnit rule failed as empty.
• testcontainers 1.21.0 -> 1.21.4. 1.21.0 pins Docker API version 1.32, and Docker 29 requires at least 1.40, so the MongoDB tests failed with Could not find a valid Docker environment.

Verification

• ./gradlew build succeeds. All 4971 tests pass, including the MongoDB testcontainers tests.
• ./gradlew jlink succeeds and produces the image.

Gradle reports remaining deprecations that are scheduled for removal in Gradle 10 (multi-string dependency notation, by project / by extra delegates, tasks.create). They all still work in 9.x and are left alone here to keep this change focused.