Bump to mill 1.1.0

[Emin017]

Type of change: other enhancement

After the release of Mill 1.1.0, which includes com-lihaoyi/mill#6655, the rocket-chip series of projects can now be migrated to Mill 1.1.0.

This pull request refactors the build configuration for the rocket-chip project.

• Rename build.sc to build.mill and update the file to use Mill 1.1.0 features, including some new API migration

This PR depends on the changes in the following PRs:

• Bump to mill 1.1.0 diplomacy#24
• Migrate from Mill 0.11 to 1.1.0 cde#33
• Simplify plugin module reference in HasScalaPlugin trait chisel#5190
• Use moduleDir to locate circt json chisel#5187
• Bump to mill 1.1.0 ucb-bar/berkeley-hardfloat#89