[improve][build] Upgrade Gradle to 9.7.1 - #26440

Motivation

Gradle 9.7.1 is a patch release that fixes six
regressions introduced in 9.7.0, the version this repository upgraded to in #26332.

The one that matters most for day-to-day development is
gradle/gradle#38788: 9.7.0 broke the formatting of
test failure comparisons, which disabled the "Click to see difference" IDE support for failed
assertions. 9.7.1 restores it.

The other five: BaseExecSpec stream API conformance, ANTLR leaking onto the kapt compile classpath,
custom Transformer implementations working again, parent-first class resolution for ant.taskdef
with an explicit classpath, and the required argument order of Option annotations in the Kotlin DSL.

Modifications

Bump distributionUrl and distributionSha256Sum in gradle/wrapper/gradle-wrapper.properties to
9.7.1. The checksum matches the published gradle-9.7.1-bin.zip SHA-256 (acd53f1e…).

gradlew, gradlew.bat and gradle-wrapper.jar need no regeneration: the published
gradle-9.7.1-wrapper.jar has the same SHA-256 (7a9ce74c…) as the 9.7.0 jar already checked in, and
./gradlew wrapper --gradle-version 9.7.1 leaves all three files untouched. That task still rewrites
retries/retryBackOffMs to its own defaults, so those are kept at the values this repository uses
(3 / 1000).

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

./gradlew spotlessCheck checkstyleMain checkstyleTest passes locally on 9.7.1.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Build tooling only: the Gradle wrapper, 9.7.0 → 9.7.1. No change to any shipped artifact.