Consider raising minimum supported Gradle version to 8

[ciscoo]

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The current required minimum Gradle 6.1.1 which was released nearly 4 years ago: https://docs.gradle.org/6.1.1/release-notes.html

I think its time to raise the minimum to a supported version of Gradle. By supported, I mean the Gradle team has a milestone for it: https://github.com/gradle/gradle/milestones

There exists a milestone for Gradle 7, but there have been several release of 8 already, I suggest going for the latest.

I'm not familiar with Spotless' codebase, but I would think this would reduce some technical debt due to maintaining support for legacy versions of Gradle. And of course take advantage of any new things.

[nedtwigg]

The only technical debt around Gradle that I am aware of is

• Petition Gradle to support JVM-local configuration-cache #987

which is not related to supporting old versions of Gradle. I'd happily bump minimum to 8 if it provided anything, and I'm happy to reopen if someone finds something where our thorough back-compat is holding us back.