Release version 2.1.0

[SethTisue]

The immediate motivation is that I want to make sure publishing still works after #416

But there's a few other little changes it doesn't hurt to get out there

• remove old publishing secrets from Travis-CI
• add new publishing secrets to GHA (under Settings -> Secrets)

===

• merge PRs
• choose version number
  • 2.1.0
• run versionCheck locally (after set every version := ...)
• push tag
  • https://github.com/scala/scala-parser-combinators/releases/tag/v2.1.0
• wait for release build
  • https://github.com/scala/scala-parser-combinators/actions/runs/1280943326
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-parser-combinators_3/2.1.0/
  • (not a complete list)
• convert the tag to a release, adding release notes
  • https://github.com/scala/scala-parser-combinators/releases/tag/v2.1.0
• any README or build.sbt edits needed?
  • post-release adjustments #424