Update scala-library to 2.12.15

[scala-steward]

Updates org.scala-lang:scala-library from 2.12.13 to 2.12.15.

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.

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Have a fantastic day writing Scala!

Files still referring to the old version number

The following files still refer to the old version number (2.12.13). You might want to review and update them manually. ``` .github/workflows/ci.yml ```

Ignore future updates

Add this to your `.scala-steward.conf` file to ignore future updates of this dependency: ``` updates.ignore = [ { groupId = "org.scala-lang", artifactId = "scala-library" } ] ```

labels: library-update, semver-patch, old-version-remains

[rtar]

It is already updated to a newer version.