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Rework method of updating atomic-updated data volumes

**What this PR does / why we need it**:

This change affects the way that secret, configmap, downwardAPI and projected volumes (which all use the same underlying code) implement their data update functionality.

* Instead of creating a subdirectory hierarchy that will contain symlinks to each actual data file, create only symlinks to items in the root of the volume, whether they be files or directories.
* Rather than comparing the user-visible data directory to see if an update is needed, compare with the current version of the data directory.
* Fix data dir timestamp format year
* Create `..data` symlink even when a data volume has no data so consumers can have simplified update watch logic.

**Which issue(s) this PR fixes**:
Fixes #57421

**Release note**:
```release-note
Correct issues that arise when volumes are mounted beneath another secret, configmap, downwardAPI or projected volume
```