Update sbt-mima-plugin to 0.6.4

[scala-steward]

Updates com.typesafe:sbt-mima-plugin from 0.6.3 to 0.6.4.

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.

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 = "com.typesafe", artifactId = "sbt-mima-plugin" } ]

labels: sbt-plugin-update, semver-patch

[rossabaker]

Test failure is #2634.