Remove dependency on `sbt-dependency-graph`

[mkurz]

Turns out, this line

akka-http/project/plugins.sbt

Line 16 in 67c7041

addSbtPlugin("net.virtual-void" % "sbt-dependency-graph" % "0.10.0-RC1") // for advanced PR validation features

is not needed anymore since sbt 1.4, because it's build in now.
See https://www.scala-sbt.org/1.x/docs/sbt-1.4-Release-Notes.html#sbt-dependency-graph+is+in-sourced

Actually for advanced usage you can replace it with

addDependencyTreePlugin

However it's not that easy, because you make use of apis here:

akka-http/project/ValidatePullRequest.scala

Lines 10 to 11 in 67c7041

	import net.virtualvoid.sbt.graph.ModuleGraph
	import net.virtualvoid.sbt.graph.backend.SbtUpdateReport

which in sbt are now internal...
(Maybe it's easy to fix, but didn't try, leave that up to you)

[mkurz]

Same story here:

• Remove dependency on sbt-dependency-graph lightbend/sbt-paradox-dependencies#34