Update webhooks code samples to match the new cache

[bszyman]

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

• 15/umbraco-cms/reference/webhooks/README.md

• 15/umbraco-cms/reference/webhooks/expanding-webhook-events.md

Remember to also update the V16 counterparts:

• 16/umbraco-cms/reference/webhooks/README.md

• 16/umbraco-cms/reference/webhooks/expanding-webhook-events.md