Post API changes clean up

What does this MR do?

A series of clean up that is required post the API scan implementation:

• test/end-to-end/fixtures/open-api-v2 should be renamed to test/end-to-end/fixtures/rest-api
• Rename SpideredMessages to ScannedResources

A CHANGELOG entry has also been added in anticipation of release.

What are the relevant issue numbers?

gitlab-org/gitlab#10928 (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
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer