[18.8] GLQL advanced finder, remove project_ids

What does this MR do and why?

Backports !226352 (merged) which removes the unnecessary project_ids collection from the GLQL advanced finder.

The confidential filters were converted to use traversal_id + project_id for authorization and no longer rely upon the project ID list.

Backport notes for 18.8:

• Merge conflict resolved in ee/lib/ee/search/advanced_finders/work_items_finder.rb — removed the project_ids method that was still present in the 18.8 stable branch.

Related:

• Original MR: !226352 (merged)
• Issue: #592420 (closed)
• #592405 (closed)
• #558781 (closed)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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