Update scalastyle-sbt-plugin to 0.9.0

[scala-steward]

Updates org.scalastyle:scalastyle-sbt-plugin from 0.8.0 to 0.9.0.

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

If you'd like to skip this version, you can just close this PR. If you have any feedback, just mention @scala-steward in the comments below.

Have a fantastic day writing Scala!

Ignore future updates

Add this to your .scala-steward.conf file to ignore future updates of this dependency:

updates.ignore = [{ groupId = "org.scalastyle", artifactId = "scalastyle-sbt-plugin" }]

labels: semver-minor