[pp-1] Update TaskInterface documentation to mention key value expirable instead of state

Problem/Motivation

@catch noted that key value expirable might be better than state https://git.drupalcode.org/project/drupal/-/merge_requests/17135#note_36...
That should probably be discussed in a follow up since the docs were mostly kept the same.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet