Remove the github-dependency-graph CI workflow

[Gedochao]

https://github.com/ckipp01/mill-dependency-submission
https://github.com/ckipp01/mill-github-dependency-graph
...so it seems that the Mill dependency graph Github action + Mill plugin got archived this month.
which in turn means that it will likely not be fixed to work with Scala CLI on Mill 0.12.14
As it currently seems to be borked:
https://github.com/VirtusLab/scala-cli/actions/runs/15341396978/job/43168133692

[63/65] ================== io.kipp.mill.github.dependency.graph.Graph/submit ================= 35s java.lang.ClassCastException: class scala.Tuple2 cannot be cast to class scala.Tuple3 (scala.Tuple2 and scala.Tuple3 are in unnamed module of loader 'app') mill.resolve.Resolve$.$anonfun$invokeCommand0$1(Resolve.scala:166) mill.resolve.Resolve$.$anonfun$invokeCommand0$1$adapted(Resolve.scala:166) scala.collection.MapOps$WithFilter.$anonfun$withFilter$1(Map.scala:402) scala.collection.MapOps$WithFilter.$anonfun$withFilter$1$adapted(Map.scala:402) scala.collection.Iterator$$anon$6.hasNext(Iterator.scala:479) scala.collection.Iterator$$anon$10.hasNext(Iterator.scala:601) scala.collection.immutable.List.prependedAll(List.scala:152) scala.collection.immutable.List$.from(List.scala:685) scala.collection.immutable.List$.from(List.scala:682) scala.collection.IterableFactory$Delegate.from(Factory.scala:288) scala.collection.immutable.Iterable$.from(Iterable.scala:35) scala.collection.immutable.Iterable$.from(Iterable.scala:32) scala.collection.IterableOps$WithFilter.flatMap(Iterable.scala:903) mill.resolve.Resolve$.invokeCommand0(Resolve.scala:166) mill.resolve.Resolve$.$anonfun$instantiateCommand$1(Resolve.scala:151)

The failure can be reproduced locally with:

./mill -i --import ivy:io.chris-kipp::mill-github-dependency-graph::0.2.7 io.kipp.mill.github.dependency.graph.Graph/submit

I'm thinking we should remove it for now, and potentially re-add it if it ever gets fixed.
cc @ckipp01