Update to Gradle 9.1.0

This is to be consistent with the exercism/java-test-runner#171 which
updated the test runner to use Gradle 9.1.0.

Author: kahgoh

Dockerfile

@@ -1,4 +1,4 @@
-FROM gradle:8.7-jdk21 AS build
+FROM gradle:9.1.0-jdk25 AS build
 
 WORKDIR /app
 COPY --chown=gradle:gradle . /app

gradle/wrapper/gradle-wrapper.jar

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.7-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.1.0-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME