pv+pvc: use API enum values for state constants

[rwe]

Description

Minor internal cleanup to the persistent_volume and persistent_volume_claim resources.

This updates literal strings to refer to their enum definitions, which makes it easier to trace back to the documentation and verify; avoids typos; and makes it straightforward to identify newly available values.

No functional change.

Release Note

Release note for CHANGELOG:

NONE

Community Note

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