Roll 2.8.0 release

[SethTisue]

@lrytz suggested it's time

v2.7.0...main

[SethTisue]

There are additions, not just fixes, so it should be 2.8.0.

[SethTisue]

Dropping support for Scala 3.0.x.

[SethTisue]

The release job seems to have done its thing: https://github.com/scala/scala-collection-compat/runs/7259729284

[SethTisue]

Artifacts are on Maven Central.

The following smoke test succeeded: scala/scala-parser-combinators#473

[SethTisue]

https://github.com/scala/scala-collection-compat/releases/tag/v2.8.0