chore: use constants for metadata

[bdovaz]

• Resolves: Use constants in the names of the two fields that are currently readable or writable in file versioning #52088

Summary

Use constants for file_versions metadata

TODO

• ...

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)

[bdovaz]

Why does it fail? It doesn't seem to have anything to do with my changes, does it?

[provokateurin]

No, cypress always fails if the PR comes from a fork. The other CI runs are just flaky in this case, so I'll restart them.

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

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