Drupal 7 date fields configured to not collect the hour/minute/second granularities can have a different "settings" structure than the migration assumes

Problem/Motivation

As shown in #2699895: Add settings to define #date_increment for datetime and datetime_range 'default' widgets, the Drupal 7 date module allowed the site builder to configure which granularity to collect:

For example, on my personal site, I have a project NodeType which has a "time range" date field that only cares about year + month granularity. Example: https://wimleers.com/work/project/cdn-far-future-expiration-drupal-7 and https://wimleers.com/work/project/ledgrid.

When migrating this to Drupal 8, \Drupal\datetime\Plugin\migrate\field\DateField::defineValueProcessPipeline() chokes on this. This causes big warnings even before migrating/upgrading, i.e. upon going from /upgrade to /upgrade/review.

Furthermore, it seems that the migration expects those granularities to be keys, but on my site they're values? 🤷‍♂️

Proposed resolution

Make \Drupal\datetime\Plugin\migrate\field\DateField::defineValueProcessPipeline() more robust.

Remaining tasks

Commit.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

TBD