Publish 2.0.0-RC1

[SethTisue]

main motivation: support Scala 3.0.0-RC2

more context: #432

• merge PRs
  • add Scala 3.0.0-RC2 and JDK 16, drop M3 and 15; move to sbt 1.5 #503
• choose version number: 2.0.0-RC1 (as per Release 2.0.0 (RC, then final) #432)
• create the release and write the notes
  • https://github.com/scala/scala-xml/releases/tag/v2.0.0-RC1
• wait for release build
  • https://travis-ci.com/github/scala/scala-xml/builds/221549259
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-RC2/2.0.0-RC1/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-RC1/2.0.0-RC1/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.13/2.0.0-RC1/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/2.0.0-RC1/
  • (not a complete list)
• post-release, bump version in README and build.sbt
  • update build and readme now that 2.0.0-RC1 is out #504