Add a check for handling updates to existing fields to decrease the minimums

[rashmigottipati]

This builds on top of #666 and focuses on adding a valid check to ensure that minimums can be decreased for existing fields in a CustomResourceDefinition schema.

If the proposal for adding this functionality to carvel-dev/kapp has been accepted prior to starting this work, all changes should be made against carvel-dev/kapp, otherwise the changes should be made against https://github.com/everettraven/kapp/tree/feature/crd-upgrade-safety-preflight

[everettraven]

The proposal has merged and work is now being tracked in carvel-dev/kapp. This issue is now a tracker for carvel-dev/kapp#913

[everettraven]

Closing as carvel-dev/kapp#913 has been closed via a merged PR.