Update mill-main to 0.9.11

[scala-steward]

Updates com.lihaoyi:mill-main from 0.9.10 to 0.9.11.
GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

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Files still referring to the old version number

The following files still refer to the old version number (0.9.10).
You might want to review and update them manually.

build.sc
readme.adoc

Ignore future updates

Add this to your .scala-steward.conf file to ignore future updates of this dependency:

updates.ignore = [ { groupId = "com.lihaoyi", artifactId = "mill-main" } ]

labels: library-update, early-semver-minor, semver-spec-patch, old-version-remains, commit-count:1