Release version 2.6.0

[SethTisue]

• merge PRs
• choose version number
  • 2.6.0
• run set every version := "2.6.0"; versionCheck locally
• create the release
  • https://github.com/scala/scala-collection-compat/releases/tag/v2.6.0
• wait for release build
  • https://github.com/scala/scala-collection-compat/runs/4148617755
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_3/2.6.0/
  • (not a complete list)
• any post-release README edits needed?

[SethTisue]

sbt-version-policy wanted this to be 2.6.0 not 2.5.1 so I went along with that.

[SethTisue]

fyi @lrytz

[lrytz]

Thanks!