Roll a release (2.9.0)

[SethTisue]

let's roll a release after #566 and #569 land

[SethTisue]

• tag: https://github.com/scala/scala-collection-compat/releases/tag/v2.9.0
• release job: https://github.com/scala/scala-collection-compat/actions/runs/3565816763
• sample artifacts on Maven Central: https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.12/2.9.0/