Update Delivery API code samples to match the new cache

[bszyman]

Redo the following V15 code samples, so they do not use IPublishedSnapshotAccessor:

• 15/umbraco-cms/reference/content-delivery-api/additional-preview-environments-support.md [Note from Mike: handled in issue Content Delivery API > Additional Preview Environments Support #7426]

• 15/umbraco-cms/reference/content-delivery-api/custom-property-editors-support.md

Remember to also update the V16 counterparts:

• 16/umbraco-cms/reference/content-delivery-api/additional-preview-environments-support.md [Note from Mike: handled in issue Content Delivery API > Additional Preview Environments Support #7426]

• 16/umbraco-cms/reference/content-delivery-api/custom-property-editors-support.md