Remove dependency on `sbt-dependency-graph`

Turns out, this line

https://github.com/apache/incubator-pekko-http/blob/a6f2ff64a8a028b39a662b460da1ba4c1fe81105/project/plugins.sbt#L20

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:
https://github.com/apache/incubator-pekko-http/blob/a6f2ff64a8a028b39a662b460da1ba4c1fe81105/project/ValidatePullRequest.scala#L17-L18

which in sbt are now [internal](https://github.com/sbt/sbt/blob/v1.9.8/main/src/main/scala/sbt/internal/graph/model.scala#L67)...
(Maybe it's easy to fix, but didn't try, leave that up to you)

Start with the dependency declaration in project/plugins.sbt and the API usage in project/ValidatePullRequest.scala. Check how the sbt dependency-graph functionality is used there and determine the compatible replacement for the now-internal APIs. Done means the external dependency is removed while the pull-request validation and build continue to work.

Written by the indexing model from the issue text.