Update Scala 2 CONTRIBUTING.md to reference new Scala standard library process

[SethTisue]

and README.md too, perhaps?

as per https://www.scala-lang.org/blog/2026/02/05/standard-library-process.html and https://github.com/scala/scala3/blob/main/docs/_docs/contributing/procedures/contributing-to-stdlib.md

[som-snytt]

What is the process by which we sneak stuff in and no one is the wiser?

[SethTisue]

@som-snytt shhhhhhh