Bump the gradle plugin major version to reflect the breaking change.

Author: nedtwigg

plugin-gradle/CHANGES.md

@@ -5,6 +5,9 @@ We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (
 ## [Unreleased]
 ### Fixed
 * Make `KtfmtConfig.ConfigurableStyle#configure` public. ([#1926](https://github.com/diffplug/spotless/pull/1926))
+* **BREAKING CHANGE** `6.23.0` made breaking changes to the ABI of the `KotlinExtension` and `GroovyExtension`. Those are reflected retroactively now.
+  - Previously, we had done semver on the Gradle plugin based only on buildscript compatibility.
+  - From now on, we will consider ABI for the benefit of convention-based plugins.
 
 ## [6.23.2] - 2023-12-01
 ### Fixed