chore(deps): update plugin com.diffplug.spotless to v6.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.diffplug.spotless	6.18.0 -> 6.21.0

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[renovate]

⚠ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: settings.gradle

Command failed: docker run --rm --name=renovate_a_sidecar --label=renovate_a_child --memory=3584m -v "/tmp/worker/5af499/428349/repos/github/diffplug/spotless":"/tmp/worker/5af499/428349/repos/github/diffplug/spotless" -v "/tmp/worker/5af499/428349/cache":"/tmp/worker/5af499/428349/cache" -e GRADLE_OPTS -e CONTAINERBASE_CACHE_DIR -w "/tmp/worker/5af499/428349/repos/github/diffplug/spotless" ghcr.io/containerbase/sidecar:9.19.3 bash -l -c "install-tool java 17.0.8+7 && ./gradlew --console=plain -q :dependencies :lib:dependencies :lib-extra:dependencies :plugin-gradle:dependencies :plugin-maven:dependencies :testlib:dependencies --update-locks com.diffplug.spotless:com.diffplug.spotless.gradle.plugin"

FAILURE: Build failed with an exception.

* Where:
Build file '/tmp/worker/5af499/428349/repos/github/diffplug/spotless/lib-extra/build.gradle' line: 73

* What went wrong:
A problem occurred evaluating project ':lib-extra'.
> 'void dev.equo.ide.CatalogDsl$TransitiveAwareList.putInto(dev.equo.solstice.p2.P2Model, dev.equo.ide.IdeHook$List)'

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 25s

[Goooler]

Seems need to merge #1758 first.

[renovate]

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