Update FAQ answer to "how to change column type" to mention column mapping

[scottsand-db]

Currently, https://docs.delta.io/latest/delta-faq.html#how-can-i-change-the-type-of-a-column says that dropping a column requires rewriting the table. This is no longer true if column mapping is enabled. https://docs.delta.io/latest/versioning.html#column-mapping. We should update this answer

We should also add another FAQ "How to drop a column".