Update sbt-idea-plugin to 3.21.0

[scala-steward]

About this PR

📦 Updates org.jetbrains:sbt-idea-plugin from 3.18.3 to 3.21.0

📜 GitHub Release Notes - Version Diff

Usage

✅ Please merge!

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 me in the comments below.

Configure Scala Steward for your repository with a .scala-steward.conf file.

Have a fantastic day writing Scala!

⚙ Adjust future updates

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

^{labels: sbt-plugin-update, early-semver-minor, semver-spec-minor, commit-count:1}

[scala-steward]

Superseded by #684.