Publish for Scala 3.0.0-RC1

[SethTisue]

after #429 lands

Main motivation:

• support Scala 3.0.0-RC1

Steps:

• merge PRs
• choose version number
  • 2.4.2. I wondered if the Scala.js version bump meant we ought to bump the minor version since it forces downstream projects to also upgrade. But I don't think I've seen other projects treating it that way?
• create the release and write the notes
  • https://github.com/scala/scala-collection-compat/releases/tag/v2.4.2
• wait for release build
  • https://travis-ci.com/github/scala/scala-collection-compat/builds/217310518
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-RC1/2.4.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-M3/2.4.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.13/2.4.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.12/2.4.2/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.11/2.4.2/
  • (not a complete list)
• post-release, bump version in README
  • 0a73146
  • 6d2c22b