Update case-app_2.13 to 2.1.0-M15

[scala-steward]

Updates com.github.alexarchambault:case-app_2.13 from 2.1.0-M14 to 2.1.0-M15.
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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Files still referring to the old version number

The following files still refer to the old version number (2.1.0-M14).
You might want to review and update them manually.

website/docs/release_notes.md

Adjust future updates

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

updates.ignore = [ { groupId = "com.github.alexarchambault", artifactId = "case-app_2.13" } ]

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

dependencyOverrides = [{
  pullRequests = { frequency = "@monthly" },
  dependency = { groupId = "com.github.alexarchambault", artifactId = "case-app_2.13" }
}]

labels: library-update, early-semver-pre-release, semver-spec-pre-release, old-version-remains, commit-count:1