Update sbt-sonatype to 3.9.21

[xerial-bot]

Updates org.xerial.sbt:sbt-sonatype from 3.9.20 to 3.9.21. GitHub Release Notes - Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

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Adjust future updates

Add this to your `.scala-steward.conf` file to ignore future updates of this dependency: ``` updates.ignore = [ { groupId = "org.xerial.sbt", artifactId = "sbt-sonatype" } ] ``` Or, add this to slow down future updates of this dependency: ``` dependencyOverrides = [{ pullRequests = { frequency = "30 days" }, dependency = { groupId = "org.xerial.sbt", artifactId = "sbt-sonatype" } }] ```

labels: sbt-plugin-update, early-semver-patch, semver-spec-patch, commit-count:1