Remove Bitbucket Cloud wiki import code

What does this MR do and why?

Atlassian removed Bitbucket Cloud wikis on 2026-08-20. Since then, the wiki step of the Bitbucket Cloud importer can only fail. Every import tries to fetch a wiki that no longer exists, logs a warning, and moves on.

I removed that step, so Bitbucket Cloud imports no longer try to import a wiki.

One thing changes for imported projects. Before, when Bitbucket said the repository had a wiki, GitLab didn't create the usual empty wiki, because it expected to import one. Now every imported project gets the default empty wiki, like any other new project.

I also updated the docs. The Bitbucket Cloud import page no longer lists Wiki as an imported item. The API token no longer needs the read:wiki:bitbucket scope, on both the import page and the import API page.

The deprecation marks this as a breaking change for 19.5. The commit has a Changelog: removed trailer.

I only removed tests here (the ones for the wiki step). The remaining Bitbucket Cloud import tests ran in pipeline 2933366555. rspec unit predictive passed with 8 examples and 0 failures. rubocop and docs-lint markdown passed there too. Locally I only ran ruby -c on the changed Ruby files. I also ran git grep -i wiki over the importer code and its specs, and it finds nothing now. I couldn't try a real import because I don't have GDK.

I have two questions:

1. Is the default empty wiki OK for imported projects? Or should Bitbucket Cloud imports skip creating it, like Bitbucket Server imports do?
2. The Bitbucket client gem in this repository can still read whether a repository has a wiki, but nothing uses that now. Should I remove it in this MR, or in a follow-up?

References

• Resolves #601065
• Deprecation: #601061 (closed)
• #601064 removes the issue import code and edits the same docs table, so one of us will need a small rebase.

Screenshots or screen recordings

Not applicable, there are no UI changes.

How to set up and validate locally

1. Import a Bitbucket Cloud repository (New project > Import project > Bitbucket Cloud).
2. Check that the import finishes, and that the import log has no Wiki import skipped warning.
3. Check that the imported project has the default, empty wiki.

MR acceptance checklist

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Implemented with AI assistance; I reviewed and tested the changes.