Upgrade Gradle wrapper 9.6.1 → 9.7.1

[serhiy-bzhezytskyy]

The wrapper is on 9.6.1 (from #599); 9.7.1 is the current stable release.

Unlike the last bump this one needs no build-script change — ./gradlew jar --warning-mode all reports zero deprecations both before and after.

Happy to open the PR.