Publish a new version (now that Scala 3 support has landed)

[SethTisue]

• merge the 3.0.0-M1 PR (Add cross build for Scala 3 #386)
• [ ] upgrade to 2.13.4
  • including bringing in 2.13.4's LazyList changes
• choose version number
  • 2.2.0 -> 2.3.0 — it's not a back-publish or a bugfix, because @unused landed (add @unused. #373)
• tag the release on github, including notes
• release build: https://travis-ci.com/github/scala/scala-collection-compat/builds/203248498
• release the staging repos
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3.0.0-M1/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.13/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-migrations_2.12/
  • (and others, of course)
• post-release PR
  • bump version numbers, now that 2.3.0 is out #391 (last time: update readme and build for 2.1.3 release #282)

[SethTisue]

not upgrading to 2.13.4, since 3.0.0-M1 still has the 2.13.3 standard library and there isn't an M2 yet: #389