KAFKA-19756 Add documentation for Gradle version upgrade process (#20741)

Currently, there is no documentation describing how to upgrade Gradle in
the Kafka project. The upgrade process involves   updating three
separate configuration files with specific checksums and URLs, which can
be error-prone without clear guidance.

- Added a new "Upgrading Gradle version" section to README.md
- Documents the 3-step process for upgrading Gradle:
  - Update Gradle version in gradle/dependencies.gradle
  - Update wrapper JAR checksum in wrapper.gradle
  - Regenerate the `gradle-wrapper.properties` and  the wrapper script

Reviewers: Ken Huang <[email protected]>, PoAn Yang
 <[email protected]>, Ming-Yen Chung <[email protected]>, Chia-Ping
 Tsai <[email protected]>

Author: joshua2519

README.md

@@ -271,6 +271,10 @@ only safe if the Scala library version is the same at compile time and runtime.
 may depend on the kafka jar for integration tests where they may include a scala library with a different version), we don't enable it by
 default. See https://www.lightbend.com/blog/scala-inliner-optimizer for more details.
 
+### Upgrading Gradle version ###
+
+See [gradle/wrapper/README.md](gradle/wrapper/README.md) for instructions on upgrading the Gradle version.
+
 ### Running system tests ###
 
 See [tests/README.md](tests/README.md).

gradle/wrapper/README.md

@@ -0,0 +1,42 @@
+# Gradle Wrapper
+
+This directory contains the Gradle wrapper files for the Apache Kafka project.
+
+## Upgrading Gradle version
+
+When upgrading the Gradle version used by the Kafka project, update the following files in order:
+
+1. **Update Gradle version** in `gradle/dependencies.gradle`:
+   ```groovy
+   gradle: "9.2.0"
+   ```
+
+2. **Update wrapper JAR checksum** in `wrapper.gradle`:
+   - Find the ***Wrapper JAR Checksum*** at https://gradle.org/release-checksums/
+   - Update the `wrapperChecksum` variable:
+   ```groovy
+   task bootstrapWrapper() {
+       // ... (other code)
+       doLast {
+           // ... (other code)
+           String wrapperChecksum = "<wrapper-jar-sha256-checksum>"
+           // ...
+       }
+   }
+   ```
+   - Verify the wrapper JAR URL is accessible at:
+     `https://raw.githubusercontent.com/gradle/gradle/v<gradle-version>/gradle/wrapper/gradle-wrapper.jar`
+
+
+3. **Regenerate the `gradle/wrapper/gradle-wrapper.properties` and the wrapper script**:
+   - Find the ***Binary-only (-bin) ZIP Checksum*** for the binary distribution at https://gradle.org/release-checksums/
+
+      ```shell
+      ./gradlew wrapper --gradle-version <gradle-version> \
+      --distribution-type bin \
+      --gradle-distribution-sha256-sum <binary-distribution-checksum>
+      ```
+
+After upgrading, verify the Gradle version:
+
+    ./gradlew --version