Update scala-library to 2.12.18

[http4s-steward[bot]]

Updates org.scala-lang:scala-library from 2.12.15 to 2.12.18. GitHub Release Notes - Version Diff

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!

Adjust 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" } ] ``` Or, add this to slow down future updates of this dependency: ``` dependencyOverrides = [{ pullRequests = { frequency = "30 days" }, dependency = { groupId = "org.scala-lang", artifactId = "scala-library" } }] ```

labels: library-update, early-semver-patch, semver-spec-patch, commit-count:1

[http4s-steward[bot]]

Superseded by #264.