Go back a bit to support all versions of CI:

From 6.13.0 to 6.14.0, Spotless casually dropped Java 8 support:
https://github.com/diffplug/spotless/blob/main/plugin-gradle/CHANGES.md#6140---2023-01-26

What is a major change, if inability to use is not 🙄?

build.gradle.kts

@@ -7,7 +7,9 @@ plugins {
     `kotlin-dsl`
     `maven-publish`
     id("com.gradle.plugin-publish") version "0.21.0"
-    id("com.diffplug.spotless") version "6.15.0"
+    // From 6.14.0 onwards Spotless requires Gradle to be on Java 11,
+    // but we still use Java 8 in .github/workflows/java-versions.yml.
+    id("com.diffplug.spotless") version "6.13.0"
     id("com.github.johnrengelman.shadow") version "6.1.0"
     id("org.jetbrains.gradle.plugin.idea-ext")
     id("com.github.ben-manes.versions") version "0.39.0"