Publish 2.0.0-M5

[SethTisue]

main motivation: support Scala 3.0.0-RC1

• merge PRs
  • support Scala 3.0.0-RC1 (and drop 3.0.0-M2) #493
• choose version number: 2.0.0-M5
  • there aren't really any code changes, but the Scala.js upgrade to 1.5.0 is a substantive change since Scala.js versions aren't forward compatible, so I think that merits a patch-level bump
• create the release and write the notes
  • https://github.com/scala/scala-xml/releases/tag/v2.0.0-M5
• wait for release build
  • https://travis-ci.com/github/scala/scala-xml/builds/217315649
• 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-RC1/2.0.0-M5/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-M3/2.0.0-M5/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/2.0.0-M5/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.13/2.0.0-M5/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_native0.4_2.12/2.0.0-M5/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_native0.4_2.13/2.0.0-M5/
  • (not a complete list)
• post-release, bump version in README
  • 38a0192