Update Mill build to 1.1.4

[Copilot]

Migrates the build infrastructure from Mill 0.12.2 to 1.1.4, adopting the new file conventions and dependency syntax.

Build file changes

• build.sc → build.mill; deps.sc → deps.mill
• Version pinning moved from .mill-version to //| mill-version: 1.1.4 header in build.mill
• import $ivy. → import $dep. for plugin imports
• ivy"..." → mvn"..." for all dependency literals

Plugin updates

Plugin	Old	New
mill-mima	0.0.24	0.2.1 (Mill 1.x-compatible)
mill.vcs.version	0.4.0	0.4.1

mimaPreviousArtifacts override removed — mill-mima 0.2.x derives it from mimaPreviousVersions by default.

Launcher scripts

mill and mill.bat replaced with the official Mill 1.1.4 launchers, which support the //| mill-version: directive and download from Maven Central.