Release Dotty 0.9.0-RC1 and 0.8.0

[allanrenucci]

• On branch0.8.x, set baseVersion to 0.8.0
• Tag 0.8.0. This will publish artefacts to Sonatype and GitHub Release
• Merge branch 0.8.x into master
• Create branch 0.9.x
• On 0.9.x, set baseVersion to 0.9.0-RC1
• Tag 0.9.0-RC1. This will publish artefacts to Sonatype and GitHub Release
• Add release notes to the GitHub Release
• On master, set baseVersion to 0.10.0
• Update scalaVersion (and, if applicable, the sbt-dotty version) in the example projects
  • https://github.com/lampepfl/dotty-example-project
  • https://github.com/lampepfl/dotty.g8
  • https://github.com/lampepfl/dotty-cross.g8
  • https://github.com/lampepfl/homebrew-brew
  • https://github.com/lampepfl/packtest
  • https://github.com/scalacenter/scastie/ 🚫 Scastie uses sbt 0.13
• Make an announcement thread on https://contributors.scala-lang.org
• Publish Blog Post on dotty.epfl.ch
• Tweet the announcement blog post on https://twitter.com/scala_lang

[allanrenucci]

Waiting for #4648 and #4490