Refined BackgroundCleanupJob queries for robustness, allowing CACHEDIR.TAG in preview root

[dennisverspuij]

I personally like NextCloud AIO to exclude previews from backup, by adding a appdata_*/preview/CACHEDIR.TAG file.
However the queries of the \OC\Preview\BackgroundCleanupJob (may) fail because of this,
This PR fixes this by making the queries more precise.

[kesselb]

Related: #42540

Haven't tested it but looks reasonable.

I am not sure if I like the idea of having unrelated files there, but I see the use case.

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