Aiming to cut Mill 0.12.9 early 17 March 2025

[lihaoyi]

Lots of good stuff has landed in 0.12.x, including

• Use process pool for test runner (0.12.x) #4679
• Backport: Improve performances of Bloop/install (#4600) #4628
• Break the dependency between repositoriesTask and ivyDeps #4472

And soon

• Support mill init from an existing sbt project, and improve related Maven and Gradle support #4586

We've been rebootstrapping using development releases internally, once all this stuff has landed it's a good time to cut a new stable version