KAFKA-20851 spotless gradle plugin version update: 8.4.0 -->> 8.9.0

[dejan2609]

See KAFKA-20851 for more details (and testing procedure).

Note: verified locally:

• ./gradlew build -x test passes (spotlessCheck + checkstyle + spotbugs + compile all green)
• ./gradlew clean spotlessApply --rerun-tasks --no-build-cache ran 700 times with 0 failures

Testing ideas are borrowed from these PR's:

• KAFKA-20182: Upgrade gradle to 9.4.1 and spotless to 8.3.0 #22012
• KAFKA-20182: Remove removeUnusedImports and upgrade spotless to 8.4.0 #22025

Release notes links:

• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.9.0
• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.8.0
• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.7.0
• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.6.0
• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.5.1
• https://github.com/diffplug/spotless/releases/tag/gradle%2F8.5.0

[dejan2609]

Build results (testing in a loop for a 700 times): ✅

BUILD SUCCESSFUL in 4s
207 actionable tasks: 207 executed
Consider enabling configuration cache to speed up this build: https://docs.gradle.org/9.4.1/userguide/configuration_cache_enabling.html
**************************************************************************
### Build succeeded (execution number: 701) -->> running again... ###
**************************************************************************

> Configure project :
Starting build with version 4.4.0-SNAPSHOT (commit id 157bbd19) using Gradle 9.4.1, Java 25 and Scala 2.13.18
Build properties: ignoreFailures=false, maxParallelForks=8, maxScalacThreads=8, maxTestRetries=0
│███████████████│ 100% CONFIGURING [1s]
> IDLE
^C>>> Build failed on execution number: 702 at Tue 28 Jul 14:43:45 CEST 2026 <<<
dejan@dejan:~/kafka$ cat repeatUntilBuildFails.sh 
#!/bin/bash

counter=0
while ./gradlew clean spotlessApply --rerun-tasks --no-build-cache; do
    ((counter++))
    echo "**************************************************************************"
    echo "### Build succeeded (execution number: $counter) -->> running again... ###"
    echo "**************************************************************************"
done

((counter++))
echo ">>> Build failed on execution number: $counter at $(date) <<<"
dejan@dejan:~/kafka$ date
Tue 28 Jul 14:44:06 CEST 2026
dejan@dejan:~/kafka$
dejan@dejan:~/kafka$ cat repeatUntilBuildFails.sh 
#!/bin/bash

counter=0
while ./gradlew clean spotlessApply --rerun-tasks --no-build-cache; do
    ((counter++))
    echo "**************************************************************************"
    echo "### Build succeeded (execution number: $counter) -->> running again... ###"
    echo "**************************************************************************"
done

((counter++))
echo ">>> Build failed on execution number: $counter at $(date) <<<"
dejan@dejan:~/kafka$

[dejan2609]

I'll rebase (conflicts with trunk must be resolved).