Update sbt-scalajs-bundler to 0.21.0

[scala-steward]

Updates ch.epfl.scala:sbt-scalajs-bundler from 0.20.0 to 0.21.0.
GitHub 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 = "ch.epfl.scala", artifactId = "sbt-scalajs-bundler" } ]

Or, add this to slow down future updates of this dependency:

dependencyOverrides = [{
  pullRequests = { frequency = "@monthly" },
  dependency = { groupId = "ch.epfl.scala", artifactId = "sbt-scalajs-bundler" }
}]

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