Add new qa resources for new streaming code, remove legacy ones

What does this MR do and why?

Add new qa resources for new streaming code, remove legacy ones

Adds:

• qa/qa/ee/resource/group_external_streaming_destination.rb and qa/qa/ee/resource/instance_external_streaming_destination.rb

Updates:

• Existing tests for new selectors on frontend / disregard old frontend code which is no longer visible/accessible
• Verification token field -> secret token field
• Updated API calls

Changelog: other

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Pipelines pass
• https://docs.gitlab.com/charts/development/gitlab-qa/#running-gitlab-qa-from-local-machine

MR acceptance checklist

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