Fix self-edge errors

What does this MR do?

Prevent panic: simple: adding self edge [recovered] errors when parsing DOT graphs (Sbt), and when creating the dependency graph used to generate the report (yarn, and possibly others).

• Change sbt parser to skip self-edges when parsing DOT exports generated by the dependencyDot task.
• Change convert.NewGraph to skip self-edges.
  • See failing job before the change: https://gitlab.com/fcatteau/si_jrscol/-/jobs/1939057037#L37
  • See successful job after the change: https://gitlab.com/fcatteau/si_jrscol/-/jobs/1939515579

In both cases, this is needed because the graph struct being used is a simple.DirectedGraph, and this type of graph doesn't support self-edges.

Self-edges can be safely ignored because they can't be used to explain why a vulnerable dependency is part of the dependency chain.

What are the relevant issue numbers?

gitlab-org/gitlab#333254 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer