fix: Add deprecation to OC.dialogs.fileexists

[susnux]

• 📝 Documentation fix(developer): Add deprecation of OC.dialogs.fileexists to upgrade… documentation#11715

Summary

We already ship this since 28, but forgot to deprecate the legacy method.
Lets deprecate it officially to be able to get rid of it as soon as possible.

Also ref #44755

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable29

[ChristophWurst]

🔥

[github-actions]

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