Bump spotless gradle plugin to 6.23.3

[kennknowles]

This has lagged behind a lot, but we recently did a major Gradle upgrade.

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[Abacn]

project :buildSrc > com.diffplug.spotless:com.diffplug.spotless.gradle.plugin:6.23.3
> No matching variant of com.diffplug.spotless:spotless-plugin-gradle:6.23.3 was found. The consumer was configured to find a library for use during runtime, compatible with Java 8,

Spotless gradle plugin dropped Java 8 support: diffplug/spotless#1530

[kennknowles]

Got it. yes I think I tried this before :-)