Upgrade Gradle wrapper 9.6.1 → 9.7.1

[serhiy-bzhezytskyy]

Fixes #612.

Bumps the Gradle wrapper 9.6.1 → 9.7.1 (current stable) via ./gradlew wrapper --gradle-version 9.7.1. Unlike #600 this one needs no build-script change — 9.7.1 surfaces no new deprecation.

Changes (4 files):

• gradle/wrapper/{gradle-wrapper.jar,gradle-wrapper.properties}, gradlew, gradlew.bat — standard wrapper task output for 9.7.1.
• gradle-wrapper.properties also gains retries=0 / retryBackOffMs=500; those are generated defaults of the newer wrapper, not hand-edited.

Verification (local; CI here is Python-only so it won't run Gradle):

• ./gradlew --version → Gradle 9.7.1
• ./gradlew jar / ./gradlew tasks --all → BUILD SUCCESSFUL
• ./gradlew jar --warning-mode all → 0 deprecations
• wrapper jar sha256 matches Gradle's official published checksum (7a9ce74c…62c5d).

Still out of scope, as in #599: pinning distributionSha256Sum and adding wrapper-validation to CI.